HP-PN71400-1_Lightwave Signal Analyzers Measure Relative Intensity Noise

8/14/2019 HP-PN71400-1_Lightwave Signal Analyzers Measure Relative Intensity Noise

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Lightw ave Signal AnalyzersMeasure Relative Intens ity Noise

Product Note 71400-1

HP 71400C/70401CApplicat ion Se ries

Laser In tensity N oise

RIN Definition

RIN Measurement Lim its

Laser RIN Personality

The HP 71400C an d 71401Caut omatically subtract th ermal an d

shot noise from RIN measu remen ts


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Table of Con ten ts

Introduction 3

Chapter 1. Measuring Laser Intensity Noise 4

Rela t ive In tensity Noise 4

Noise Sources in a Fiber-Optic System 4

Therm al Noise 5

Shot Noise 5

Laser Int ensity Noise 6

RIN Measurement Limitat ions 6

Finding RINLaser from RINSystem 7

Sma ll Er rors May Cause Large Effects 8

Summary 9

References 10

Chapter 2 . Measure Re la t ive Intens i ty Noise w i th the HP 71400C and 71401C 11

HP 71400C an d 71401C Lightwa ve Signal Analyzers 11

The 70810B Light wave Converter 11

Noise Measur ement s 12

Mark er Noise 12

System RIN Measurements 12

Measur ement Limits of RINSystem 13

Laser RIN Measurements 13

Measur ement Limits of RINLaser 14

RIN Var iability 15

Using th e Persona lity 15

Exam ple 1: DFB 16

Exam ple 2: DFB laser 17Exa mp le 3: Mini-YAG 17

Appendix A. Electr ical-Optical Relat ionships 18

Rela t ionship Between Opt ica l and Elect r ica l Powers 18

Appendi x B. P ersonal ity Ins ta ll a ti on and Removal 19

What is a Measur ement Persona lity? 19

System Requirement s 19

Reloading the Personality from the ROM Card 19

Retrieving th e DLP Execute Key from a User Overwrite 20

Er asing a DLP from th e Inst rum ents Memory 20Reloading the Personality from th e Floppy Disk 21

Ap p e n di x C. R IN R e mo t e Co m m an d s 22

Example: 23

P age


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Introduct ion

As high-speed fiber-optic comm un icat ion becomes increa singly

importan t, so does the n eed to perform precision m easur ement s on

lasers a nd lightwave systems. Las er int ensity noise is one of th e

limiting factors in t he t ra nsmission of an alog or digital signals overfiber an d mu st be char acterized. Intensity noise, or power fluctua -

tions in the lightwave signal, reduces signal-to-noise ratios (S/N)

an d increases bit err or rates. Int ensity noise degrades performa nce

in h igh-speed direct-detection systems, th ereby increasing th e need

for r epeaters or optical a mplifiers in long distance tr an smissions.

At the sa me time, however, noise limits th e nu mber of repeat ers

th at can be added to a distribution system.

This product note describes techniques to measur e laser inten sity

noise with t he H P 71400C and 71401C lightwave signal an alyzers.

Chap ter 1 defines relative intensity n oise (RIN) and describes the

cont ributions of laser, therma l, and sh ot noise to the total system

noise. Chapter 2 describes the RIN mea sur ement fun ctions ava il-

able in th e HP 71400C. Measur ement exam ples are included tha t

illustrate RIN measurements with and without the sh ot-noise and

thermal noise contributions. Also covered is the operation of a

downloada ble program tha t deter mines th e RIN of only th e laser by

subtr acting the th erma l noise of th e ana lyzer an d the sh ot n oise of

the photodiode.


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Chapter 1Measu ring Laser Intensi ty Noise

Relative Intensity Noise

The mea sur ement of relative intensity n oise (RIN) describes the

laser s ma ximu m a vailable am plitude ra nge for signa l modulation

an d serves as a qu ality indicat or of laser devices. RIN can beth ought of as a type of inverse carrier-to-noise-ratio measur ement .

RIN is the r atio of the mean -squa re optical inten sity noise to the

squar e of th e avera ge optical power:1,2

RIN = dB/Hz ; (1)

P 2

where: is th e mean -squar e optical int ensity

fluctuat ion (in a 1-Hz ba ndwidth ) at a specified

frequency, and P is the average optical power.

The r at io of optical powers squ ared is equivalent to th e ra tio of the

detected electrical powers. Thu s, RIN can be expressed in t erms of

detected electrical powers. Equat ion 1 can be rewritten as:

N elecRIN = dB/Hz (2)

P AVG(elec)

where: Nelec is the power spectral den sity of the

photocurrent at a specific frequency, and P AVG(elec)is the a verage power of th e photocurr ent.

The ph otonic shot noise is not included in t he definition of Nelec.3 From

th is point on, the (elec) subscript will be dropped, and a ll term s will be

present ed in electrical units un less noted by an optical subscript.

Noise Sou rces in a Fiber-Optic System

A typical optical receiver consist s of a ph otodetector an d

am plification. The n oise at the receiver output results from t hr ee

fundam enta l cont ributions: laser inten sity noise primar ily due to

spontan eous light emissions; th erma l noise from t he electr onics;

an d ph otonic shot noise. The t ota l system n oise, NT(f), is t he linear

sum mat ion of these th ree noise sources.

NT(f) = NL(f) + Nq + N t h(f) W/Hz (3)

where: NL(f) is the laser intensity n oise power per H z;

Nq is th e photonic shot n oise power per H z;

N th (f) is the cont ribution of therm al noise power per Hz;

Therm al an d laser in tensity n oise vary with frequen cy. Shot noise

is a function of the intensity of light incident on the photodiode.


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Electr onic tools ar e often used t o determine lightwa ve system

param eters. But, when actual electrical values are m easured to

determ ine optical para meter s, car e must be ta ken to include the

effects of photodiode r esponsivity, th e gain an d n oise effects of anyamplifier present, and the frequency response corrections of each

element.

While it is desirable to determ ine th e total system noise, it is also

valuable to determine separ at ely the individual cont ributions of

laser, shot, an d th erma l noise and to compa re each to a t otal system

noise bu dget. A discussion of each of th ese n oise sour ces follows.

Thermal Noise

In a light wave system, th e amp lifier a nd electr onics th at follow th e

photodiode produce thermal noise (N th). Therm al noise limits th e

sensitivity of th e receiver an d restr icts th e distance between tra ns-

mitter a nd receiver in both a na log and digita l systems. Therm al

noise can be expressed in several wa ys. It is often described as a

noise factor, or n oise figur e, expressed in d B relat ive to the room-

temper at ur e lower limit of 174 dBm/Hz.

To reduce th e th erm al noise cont ribu tion of th e receiver, very low

noise amplifiers ar e often added after t he ph otodiode. As a n

example, electrical spectru m a na lyzers a re often used in lightwa ve

measu remen ts. But as figure 1 sh ows, most electrical spectru m

analyzers have noise figures of 30 dB or higher. Adding a preampli-

fier will improve the sensitivity of the analyzer by reducing the

overall noise figure, and hence the ability of the analyzer to measure

light wave signals.4 Typical noise figures for a mplifiers ra nge from a

few dB (above 174 dBm/Hz) for na rr owband a mplifiers, 6 to 8 dB

for low-noise, wider-ban d am plifiers, to as m uch a s 15 dB.

Therefore, when choosing an amplifier, sensitivity often must be

tr aded off for ban dwidth.

Shot Noise

For a system at a given temperat ure, therma l noise is usually

const an t, but shot n oise varies with average power. Shot n oise (Nq)

is produced by the qua ntu m na tu re of photons arr iving at th e

detector, and related detection sta tistics. The noise produced is

related directly to the amount of light incident on the photodetector.

The mea n-squared n oise cur rent from th e photodetector is:

= (2qIdcB) (4)

where: q is the electron charge (1.60 x 1019 coulomb);

Idc is the curr ent out of the diode due to the avera ge

optical power input; and B is the electrical noise

bandwidth of th e measu rement (typically

normalized to 1 Hz).

Figure 1. Noise l imits in electricalsystems are improved with apreamplifier.


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The familiar equat ion P =I2RL, where RL is th e load r esistance of

th e amplifier input , is used to convert th is noise curr ent int o units

of power. Ther efore, t he s hot-noise power, Nq, in a one Hz bandwidth,

becomes:

Nq = (2qIdc)RL . (5)

For example, if th e load r esistance is 50 ohms, a photocur rent of 1 mA

will gener at e a s hot-noise power of 168 dBm /Hz (1.6 x 1017 mW).

As figure 2 shows, for each decade of photocurrent increase, the

shot-noise power will increase 10 dB. In some systems, the light

(>0 dBm) str iking the photodetector is enough t o cau se th e shot

noise to become lar ger tha n t he th erma l noise of the system .

Laser Inten sity Noise

Laser intensity n oise, NL, refers t o the noise genera ted by th e laser.

Laser int ensity noise is caused by intensity fluctua tions due

prima rily to spont an eous light emissions th at a re dependent on

str uctur al par amet ers of the laser. Operating conditions, such a s

bias level and modula tion frequen cy, also directly affect th e noise

level.5 The presence of external feedback or reflections into the

laser will increase t his n oise.6 As figure 3 sh ows, th e dominant

noise featu re of the laser intensity n oise is th e inten sity-noise

peaking at t he relaxation resonance point.

RIN Measu remen t Limitations

RIN is essentially a dyna mic ra nge measu remen t. It is a ra tio of

noise to average power. If th e avera ge power is r educed, the RIN

measurement ran ge is reduced. Figure 4 shows how measur ingRIN as a function of th e average power is limited by the th erma l

noise. If the average p ower incident on the detector becomes large

enough, th e shot n oise overcomes t he t herm al noise and becomes

th e mea surem ent limit. Th is shot-noise-limited condition will have

th e same noise value in an y lightwave system.

Figure 2. Shot noise increases w ithaverage power and can becomelarger than the thermal noise .

Figure 3. Laser intensi ty noise andthe re laxation resonance point varywith laser bias level .

Figure 4. Direct RIN measu rementsare l imited by the thermal noise ,shot noise , and total detected pow er.


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It is often valuable to determine er ror budgets for ea ch component .

This requ ires separ at ing th e laser-int ensity-noise contribut ion from

th e rest of th e system. For RIN measu remen ts of th e laser only,

th erma l and shot n oise effects become u nwa nted errors an d mu stbe rem oved. To avoid confusion, laser-componen t RIN m easu remen ts

are distinguished from system RIN measurements. RINSystem will

refer to the t otal system measurement and RINLaser will refer to the

RIN t ha t r esults from only excess noise genera ted by th e laser.

The value of the laser in tensity noise is found from equat ion 3 by subtra ct-

ing the values of shot a nd t herm al noise from th e total system n oise:

NL(f) = NT(f) Nq N th(f). (6)

When t he noise of the laser far exceeds th e shot or ther mal n oise

term s, the t otal system noise is essentially equal to the laser

intensity n oise. In su ch cases, RINLaser equals RINSystem. H owever,

as laser quality improves and t he int ensity-noise level decreases,

the effects of shot and thermal-noise sources become more

significan t in RIN mea sur ement s. The contr ibution of any one of

th e th ree noise terms will domina te if it is ap proxima tely 5 to 10 dB

larger th an th e oth er term s. As equat ion 3 shows, the total noise is

a sum ma tion of th ese three noise term s.

An example of a system dominated by laser intensity n oise is

provided by measur ement s ma de on a typical 1-mW Fa bry-Perot

laser. The total system noise (N T) was mea sur ed at 145 dBm/Hz,

th e ther mal noise (N th) was 168 dBm/Hz, an d th e shot n oise (Nq)

is 169 dBm/Hz (for I = 0.8 mA). Converting to linear terms and

subtr acting the shot and t herm al noise from the total gave the laser

intensity n oise. It was only 0.04 dB less th an th e total system noise.

For th is typical Fa bry-Perot laser, the t otal system noise is almost

exclusively composed of laser intensity noise. For shot and thermal

noise values t o contribute m ore tha n 1 d B to the t otal noise power,

th e excess laser int ensity noise would ha ve to be reduced 15 dB to

about 160 dBm /Hz. Thu s, improving th is system per forma nce only

requires th e laser performan ce be addressed.

Finding RINLaser from RINSystemRIN is th e ra tio of noise power to avera ge power. Equ ations 2 an d 3

can be used to determ ine th e value of RINLaser.

NT

NL

Nq

NthRI NSystem = = + + (7)

P AVG P AVG P AVG P AVG

Subtr acting ther mal a nd sh ot noise effects from th e total allows the

excess noise of the laser to be determined.

NL Nq N thRI NLaser = = RINSystem (8)

P AVG P AVG P AVG


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Chap ter 2 will describe measu remen ts of RINSystem and RINLaserperform ed using th e HP 71400C light wave signal an alyzer. The

ta bles in chapt er 2 sum ma rize the specified RINSystem and RINLasercapabilities of the H P 71400C lightwa ve an alyzer for various

frequency ranges. With th e correction factors a vailable in th e

an alyzer, subtra ction of the t herm al-noise and shot-noise terms will

provide up t o an additional 16 dB ran ge for m easur ement s of

RI NLaser. A downloada ble program for t he 71400C th at

au tomat ically performs t hese subtr actions a nd compu tes RINLaserwill be described in t he n ext chapt er.

Summary

Work on improving laser intensity noise cont inues. In some cases,

th e intensity n oise levels of the laser can appr oach t he n oise

limita tions of measur ement system. To evalua te laser-intensity

noise cont ributions a nd limita tions, the relative-intensity-noise

specificat ion, RIN, was developed. This m easur ement is th e ra tio of

th e laser int ensity noise to the avera ge power of th e laser, in

equivalent electrical un its. This chapter described some limits t o

measu ring th e noise cont ribution of the laser an d a discussed how

to remove th e shot-noise a nd ther mal-noise components associated

with determ ining the RIN of th e laser.

-130 -140 -160

RIN

dB/Hz

-110 -120 -150

-160

-150

-140

-120

-110

-130

-170

2.0

0.5

0.2

0.1

1.0

-170

Detected OpticalPower, mW:

RIN dB/Hz

Laser

System

Figure 5. RINLaser calculated from themeasured RINSystem, versus averagepower.


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References

1 K. Y. Lau an d A. Yar iv, Ultr a-High S peed Sem iconductor La sers ,

IEE E J . Quan tum Elect., QE-21, NO.2, pp. 121136, Februar y 1985.

2 C. M. Miller, Intensity Modulation and Noise Characterization of

High-Speed Semicondu ctor Lasers , IEE E LTS, pp. 4452, May 1991.

3 G. P. Agrawal and N. K. Dutta. Long-Wavelength Semiconductor

Laser s, pp. 248. AT&T Laborat ories, New York: Van Nostr an d

Reinh old, 1986.

4 See Application Note 150, Hewlett-Packard Publication no. 5952-

0292, an d Appen dix A of Application Note 371.

5 H. Sobol, The a pplication of microwave in light wave syst ems,

J . Light wave Techn ology, vol. LT5, pp. 293299, Ma rch 1987.

6 J.L. Gimlett and N.K. Chenug, Effects of phase-to-intensity noise

conver sion by mu ltiple reflections on gigabit-per-second DFB la ser

transmissions systems, J. Lightwave technology, vol. 7, pp. 888895,

J une 1989.


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Chapter 2Measu re Relat ive Intensi ty Noisew ith th e HP 71400C and 71401C

This chapter presents measurements of relative intensity noise (RIN)

perform ed specifically with t he H P 71400C and 71401C lightwave

signal ana lyzers th at contain t he HP 70810B lightwa ve section.1

Exam ples and limita tions ar e discussed, along with gu idelines forsetup, measurement, and operation of the two RIN measurement

functions t ha t ar e built into these lightwa ve signal an alyzers.

HP 71400C and 71401C Lightw ave Signal Analyzers

The HP 71400C an d 71401C lightwave signa l ana lyzers were

designed t o facilitat e developmen t a nd t estin g of fiber-optic commu -

nications systems a nd component s. The ana lyzers mea sur e impor-

ta nt lightwave cha racteristics such as signal stren gth, modulation

bandwidth, signal distortion, effects of reflected light, and noise.

Used with th e HP 11980A fiber-optic interferometer, th e HP 71400C

an d 71401C can also measur e linewidth, chirp, and frequency

modulat ion of single-frequ ency lasers. Th e HP 71400C operat es over

th e frequency ra nge of 100 kHz to 22 GHz, while the H P 71401C

covers the frequency range of 100 kHz to 2.9 GHz. The lightwave

signal ana lyzers ha ve int erna l correction and calibration th at allow

th em to display optical measur ements with 1 dB accur acy across

th eir full bandwidth. Both cont ain th e same featur es and functions,

including operat ion as h igh-performa nce electrical spectrum analyzers.

The 70810B Lightw ave Conve rter

The H P 70810B lightwa ve section serves as t he key component of the

HP 71400C and 71401C lightwave signal ana lyzers an d is available

in a choice of two modules t o cover t he optical wavelengt h r an ges

from 1,200 nm to 1,600 nm (standa rd m odule), or 750 nm t o 870 nm

(Option 850). As figure 6 shows, th e HP 70810B is compr ised of abroadband PIN photodiode, intern al optical at tenu at or, electrical

preamplifier for improved sensitivity, and an optical power meter.

1 The HP 70810B module is an update that allows down-loadable programs (DLP). The RIN

function discussed in this chapter is such a DLP. If you have a system with a 70810A module,

please contact your H P r epresentative for u pgrade details.

Figure 6. Block diagram of theHP 71400C lightw ave sign al analyzer.


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The optical power meter provides both digita l ann otation an d

an alog graph ical readout s. It can display informat ion in eith er

optical or electr ical u nits, indepen dent of th e un its selected for th e

tr ace display. The chosen r eference level on t he displa y is also usedas t he r eference level for both th e power bar an d th e tra ce. This

shared reference level allows for visual checks of the dynamic range

of the laser (average-power to noise) and relative modulation levels

over t he full bandwidth of the laser.

With th e power m eter bu ilt-in, there is n o need to cha nge conn ec-

tions between an extern al power meter an d th e signa l analyzer for

measurements tha t require both instruments. Not having to chan ge

conn ections rem oves conn ector u ncertaint y for relative mea sur e-

ments. The factory provides calibration at both 1300 nm and 1550 nm,

(or a t 850 n m for Option 850) selectible from t he m enu s. The power

meter a lso may be calibrated to a tra nsfer stan dar d. If adjusted, all

am plitu de readouts, including the modulation measur ement s, willreflect th e n ew r eference.

Noise Measurements

The lightwa ve signal an alyzer h as t hr ee noise fun ctions built in:

ma rker noise; RINSystem; and RINLaser. Marker noise is used to mea-

sur e the a bsolut e noise value at a selected frequency in either optical

or electrical un its. The t wo RIN functions m ake use of the built-in

average p ower m eter. Relat ive int ensity n oise of the laser describes

th e maximum am plitu de ran ge available for signa l modulat ion.

Marker Noise

Mark er noise reads t he average value of th e noise present a t th ema rker location, norma lized to a 1-Hz ban dwidth. This measu re-

ment is selected u sing MKNOISE on pa ge 2 of the mark er menu .

For mea sur ing a lightwave signal, eith er optical un its, referenced

to th e optical inpu t, or electrical un its, referenced to th e output of

th e photodiode, ar e available. Marker noise adjusts for equ ivalent

noise bandwidth , incorporat es proper sa mple-detection meth ods,

averages 32 points around the marker, and normalizes the result

to a 1-Hz ban dwidth.

System RIN measurements

RI NSystem the measurement that includes excess laser intensity

noise, the detection sh ot noise, and t he th erma l noise of the inst ru -

ment is pr ovided in th e HP 71400C or 71401C by th e functionRIN S YS , found in th e mar ker m enu. This function ut ilizes the

built-in power meter, amplifier, lightwave corrections, and the

ma rker-noise function t o compute t he r at io of th e total noise and

average optical power. The a nswer is ret ur ned a s th e rat io of

equivalent electrical terms , as in equ ation 2 of chapter 1, regardless

of which u nits a re displayed. For example, in figur e 7 th e an alyzer

shows both th e average power readout a nd t ra ce display in optical

units. Pr essing RIN S YS gave 141 dB/Hz, the electrical ratio.

2 For more information on noise measurements see Application Note 150, pp 3134.

Figure 7. RINSystem display.


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To use this function, set u p th e display for the desired ba ndwidth

an d ma rker location before pr essing RIN S YS . After engaging th e

function, th e mar ker is active and it ma y be moved to determ ine

RI NSystem at various locations in th e noise response of the laser.

A wide span is useful for viewing the full response of the laser and

for det ermining frequencies for optimum system operat ion a nd

signal-to-noise. However, reducing the span to about 400 MHz,

cent ered ar ound a frequen cy location of int erest, can r eveal the

presence and degree of reflection ripples (Fabry-Perot resonances)

in the system.3 Reducing th e video bandwidth (VID BW) will help

determ ine the a verage noise value. This mar ker function is usua lly

adequa te for deter mining RIN of most Fa bry-Perot lasers.

Measureme nt Limits of RINSystemWith a 0-dBm (1 mW) laser, RIN SYS is limited to about 145 dB/Hz.4

The avera ge CW power of th e laser determ ines th e upper limit.

Thus, the m easurement l imit ofRIN S YS is determined by th e

available power of the laser an d t he n oise floor (therma l plus sh ot)

of th e specific an alyzer u sed. It is best to mea sur e th e noise floor of

th e actua l ana lyzer (to avoid th e guar d-ban ds necessary to account

for wide t emperat ure ra nges an d other module-to-module variations).

However it is possible to use the published sensitivity specifications

for th e ana lyzer to determine a gener ic RIN mea surem ent limit.

At th e limit, th e system can r eceive up t o 3 dBm of optical power

before requiring attenuation. Although the internal attenuator

allows power levels up to 30 dBm, adding a tt enua tion decreases t he

sens itivity of th e an alyzer. Thus, with 3 dBm of optical power on

th e photodiode, and th e att enua tor set to 0 dB, the HP 71400C

achieves its best RIN ra nge, determined from th e data sheet, of

149 dB/Hz (including the shot noise term). Table 1 lists various

HP 71400C specificat ion limits in th e th ree key frequency ran ges

for t hree a verage power values.

Frequency Range +3 dBm Power 0 dBm Power 5 dBm power

10 MHz100 MHz 141 dB/Hz 135 dB/Hz 125 dB/Hz100 MHz16 GHz 149 dB/Hz 143 dB/Hz 133 dB/Hz16 GHz22 GHz 137 dB/Hz 131 dB/Hz 121 dB/Hz

Laser RIN Measu remen ts

RINLaser

is th e ra tio of the laser noise to average power, with t he sh ot

an d th erma l noise term s removed. Since all receivers ha ve shot and

th erma l noise, RINLaser is determined by subtr acting the excess

noise terms, as described in chapt er 1. The HP 71400C an d 71401C

provides this capability with a RIN m easur ement personality (one

of severa l downloada ble program s (DLPs) available from H P). While

th e RIN persona lity is built into th e system, it is also provided on a

ROM card an d a disk, for backup in case the program is erased.5

3 For more information see Application Note 371, chapter 3.4 Based on displayed a verage noise specifications of the HP 71400C.5 See appendix B for instructions on loading a DLP into the MMS system.

Table 1. RINSystem l imits for theHP 71400C from system s pecification


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Measureme nt Limits of RINLaserMeasurements made with t he RIN DLP ar e subject to the sa me basic

limits as t he RIN S YS function: avera ge power an d th e noise floor

of th e an alyzer. However, the DLP subt ra cts th e cont ribut ions of shotan d th erma l noise up t o a limit of 16 dB over th e value obta ined with

the RIN S YS fun ction. The limit is imposed because sma ll number s

ar e subtra cting from small num bers. An err or in one term can cause

disproportionat ly lar ge errors in th e results if pushed beyond t he

capabilities and calibrat ion of the an alyzer. If th is limit is encount ered,

a super script + symbol will be displayed after th e RIN (laser ) value,

indicating that the instru ment h as reached its measurement l imits

and th e RINLaser is this displayed value or bett er. Even with th is limit

placed on the m easur ement , there can be considerable accur acy

var iat ions (36 dB is not un comm on at low RIN valu es). Table 2 gives

possible RINLaser values, calculat ed from specificat ion limit s.6

Frequency Range +3 dBm Power 0 dBm Power 5 dBm power

10 MHz100 MHz 157 dB/Hz 151 dB/Hz 141 dB/Hz100 MHz16 GHz 165 dB/Hz 159 dB/Hz 149 dB/Hz16 GHz22 GHz 153 dB/Hz 147 dB/Hz 137 dB/Hz

The a ctu al value of these limits will var y from syst em t o system,

based on actual thermal-noise values. For example, a best case module

with an actual thermal-noise floor of 166 dBm/Hz could have a

RI NLaser limit of 170 dB/Hz (with th e 3-dB maximu m ph otodiode

illumina tion). Lower avera ge power input s will corr espondingly lower

th e RIN measu remen t capability. For the m odule in t his example, a

laser with 0 dBm inpu t power will lower th e RIN measu remen t

limit t o 165 dBm. The graph in figure 8 su mma rizes th ese measure-

ment limits. The graph plots th e cha racteristic RIN persona lity

limits (calculated) along with the RIN S YS limits a s a fun ction of

average power. It also plots t he t herm al an d shot n oise contr ibutions.

6 Specifications su bject t o chan ge.

Table 2. RINLaser l imits , using theRIN person ality, in the HP 71400Cfrom system s peci f ications

Figure 8. Typical HP 71400C mea-surements l imits for RINSystem an dRI NLaser, given a no iseless las er.


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value without the ther mal-noise or shot-noise terms. The second

line provides the RINSystem value, which is th e same nu mber tha t

the RIN S YS fun ction (under th e mar ker m enu) would give. The

th ird and four th lines present t he th erma l-noise an d shot-noisecontributions that are subtracted to obtain the Laser RIN. These

four t erms a re th e same as t hose in equa tion 11, cha pter 1. Note

th at a lthough th e tota l shot noise increases with m ore avera ge

power, the sh ot noise term decreases.

The program is not a simple fun ction; cha nging any pa ra meter will

halt the program a nd the RIN key will go from t he on state to the

off sta te. However, the DLP maint ains th e window setup. The

sta rt, stop, marker, and at tenu at ion keys are rea dily available from

th e DLP men u. If you wish to use oth er functions, press t he MENU

key, select a nd execut e the desired function, an d th en pr ess the

USER key again. Reactivate t he RIN Laser calculations a gain by

pressing th e RIN on key. If you want to exit the DLP, press theexi t key (also in t he DLP menu ). The RIN function will termina te

an d the display will retu rn t o norma l.

The personality indicates the sta te of th e measur ement values by

brightness. As the DLP is first activated, the da ta values will be

blank. As soon as th e progra m a cquires sufficient mea sur ement

informa tion, the values will appear in th e window, with t he

RI NLaser value displayed brighter th an t he cont ributing term s. If

th e RIN program is h alted (by pressing an y key, or chan ging the

mark er), the da ta update is stopped and a ll numeric values dim.

The last m easured values remain displayed unt il either th e RIN

measu remen t is turn ed back on, an d sufficient dat a is againobta ined, or t he pr ogram is exited.

The RIN persona lity operat es by taking two sweeps, one with t he

laser on a nd one with the light from th e laser blocked. The first

sweep measu res th e average power a nd t otal noise of the laser. This

is displayed in trace A. The second sweep blocks the light and

measu res th e therm al noise of the a na lyzer an d places it in tra ce C.

Both procedures a re visible while the pr ogram is run ning. Halting

the program will blank trace C, leaving trace A active. A single-

measu re key can be pressed th at will cause the an alyzer to take one

sweep, update th e data window, and leave both st atic traces

displayed. Th is function is useful when a plot is desired of both

traces. Trace C may also be turned on or off from the trace menu.

Example 1: DFB Laser

Figure 11 shows the RIN DLP measu remen t resu lts for a distrib-

ut ed feedback (DFB) laser an d th e effects of attenu at ion on t he

measu remen t. The laser ha d less than 2 mW (3 dBm) of average

power, so 0 dB att enuat ion was entered. The DLP measu red th e RIN

of th e laser t o be 152 dB/Hz, while th e RINSystem was 149 dB/Hz. Figure 11. DFB laser


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17

In figure 12, we added 5 dB of optical attenuation (10 dB electrical)

to the laser. The th erma l noise rose 10 dB and th e RINSystemdegraded t o 143 dB/Hz. But, t he compu ted RIN Laser differed by

just 0.7 dB. For th is measur ement we reduced the video band widthfor th erma l noise stability which resu lted in an increase in th e

sweep time.

Example 2: Fabry-Pe rot LaserFigure 13 shows a F abry-Perot laser that was measur ed using the

RIN Per sonality. This laser ha s sufficient inten sity noise th at the

RI NSystem fun ction gives the sa me resu lts as th e RINLaser. Using the

DLP is n ot necessary. The RIN S YS mar ker function sh ould be

used rath er tha n th e DLP because it performs th e RIN measurement

mu ch faster, and it ha s th e ability to move the m ar ker location

while th e RIN measur ement is in progress.

Exa mp le 3: Min i-YAG Las er

Figure 14 shows a m easur ement of a m ini-YAG laser perform ed

using th e DLP. The laser has a large power output , requiring

optical at tenu at ion t o keep the incident light on the p hotodiode to

th e 3 dBm limit. With add ed att enua tion the shot noise is not

determ ined by the total power of th e laser, but r ath er by the total

power of the light illuminat ing th e photo diode. The DLP is able to

determ ine corr ectly th e proper sh ot-noise contr ibution an d chan ge

in therma l noise due to the att enuation, and determine the

RIN value of th e laser (up to a limit of 16 dB better t ha n t he value

foun d by RIN S YS ). Because of the low RIN value, t he r esolution

an d video band widths h ave been set to optimize the n oise

measurement stability.

Figure 12. DFB laser with 5 dBattenuation added.

Figure 13. Fabry-Perot laser

Figure 14. Mini-yag lase r


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Appen dix AElectrical-Optical Relation ship s

Conversion between optical an d electrical values mu st t ake int o

accoun t t he sp ecific valu es of resp onsivity, load r esista nce, and gain

of th e conver ter. A simple 2:1 conversion ra tio is not su ffient . The

equations presented in Table A-1 are for convenience.

Relat ionship Between Optical and Electr ical Powers

The detected output current from th e photodetector is a linear replica

of th e optical power inp ut . A 1-dB cha nge in optical power to th e

photodetector will produce a 2-dB change in the output electrical power.

Convert ing from optical to electr ical power is eas y if you know th e

responsivity a nd the load resistan ce. However, the electrical out put

power will further be offset by the gain of any amplifier present.

Table A-1 sum ma rizes these r elationsh ips.

Note that voltage responsivity, r v , is commonly given at the refer-

ence plan e of th e photodiode. However, if an a mplifier is integra l toth e converter, as in t he case of the H P 11982A, th e responsivity is

given from th e reference plan e locat ed after th e am plifier. For t hese

cases, the equa tion for responsivity should include t he gain, G v, of

the amplifier: r v = rGvRL (rath er t ha n a s given below.)

Relationship Units

Optical Power PAVG(opt) = (RL PAVG)1/2rv watts

Responsivity r = rv/RL amps/watt

Electrical Gain Gv volts/volt

Responsivity rv = r RL volts/watt(voltage )

Detected Idc = r PAVG(opt) ampsCurrent

Detected Vd = IdcRL voltsVoltage = r RLPAVG(opt)

Amplified Vdc = r GvRLPAVG(opt) volts

Voltage

Average PAVG = Idc2 RL wattsElectrical power = (Vd)2/ RL

= RL(r PAVG(opt))2

= (rvPAVG(opt))2/ RLModulatedElectrical Power PM = 1/2(rvPM(opt))2/ RL watts

Table A-1.


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Appen dix BPe rsonal ity Insta llat ion an d Removal

This appendix describes the HP 70810B laser RIN m easur ement

pers onality. It t ells how to recall th e pers onality firmk ey, if over-

written , and how to load the personality into the instr um ent from

either th e ROM card or floppy disk.

What is a Measu remen t Person ality?

A measurem ent personality is a downloada ble progra m consisting

of meas ur ement routines especially useful to a pa rt icular application.

A downloadable program (DLP) is a software routine writt en

with an external computer and downloaded (stored) int o non-

volatile RAM in an instr umen t (in this case, the HP 70900 LO).

Several DLPs will fit into instr umen t m emory simultan eously.

However, too man y insta lled DLPs can use u p t he a vailable

memory, which limits t he a bility of the an alyzer t o save inst ru ment

sta tes, setups, or tr aces. Once downloaded, the DLP can be

execut ed at the pr ess of a softkey from th e USER m enu, with out an

externa l computer.

Th e Laser RIN Pe rsonality is a DLP that m easures the relative

intensity n oise of a laser. The RIN DLP may be removed (erased) to

ma ke room for other DLPs or tr ace storage by purging the two

files tha t a re in t he system memory. If the personality is acciden-

ta lly erased, it can be reloaded into th e instr umen t; if th e key

assigned (in th e USER menu ) is overwritt en, it can be reassigned

using th e procedures described in t his appen dix.

System Requirements

To load a DLP int o the H P 71400C lightwave signal a na lyzer, an

HP 70900B local oscillator must be configured into the system. Forth e RIN DLP t o be loaded, th e HP 70900B must h ave a da te code of

910701 (Ju ly 1, 1991) or lat er. An H P 70810B light wave section

mu st a lso be configured into the system. Th e HP 70810B mu st be

operating in a slave configuration to the master HP 70900B.

Released ea rlier, HP 70810A modules do not allow DLP applications

and must be updated to permit usage with a DLP.

Reloading the P ersona lity from the ROM Card

Following are t he steps a nd key sequences used to load t he

HP 70810B laser RIN measur ement personality into the H P 71400C

from a ROM car d.

Insert th e ROM car d, face up an d ar row end first , into the slot onth e HP 70004A display/ma infra me. Press th e MENU ha rd key on

th e instr um ent. Then follow th e menu selection sequ ence:

Press Misc, more 1 of 3, Catalog & MSI.

The tr ace display will shrink an d th e list of th e cur rent

Mass S tora ge Is ass ignmen t will be given a s in figur e B-1.Figure B-1. Display when se lect ingmass storage dest ination.


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Reloading the P ersonal i ty from the Floppy Disk

The DLP ma y also be loaded into the inst rum ent m emory with a n

externa l HP-IB floppy disk dr ive and t he su pplied disk. The

following equipment is required:

HP -IB disk drive with 3.5 inch, CS80 compa tible, floppy

media, such a s the HP 9122

HP -IB cable to conn ect from th e disk dr ive to the H P 70004

or H P 70001 manframe

HP 70900 local oscillator m odule

HP 70810B lightwave module

Laser RIN P ersonality floppy disk

Find t he H P-IB address of th e disk drive. Conn ect it t o the HP -IB

socket on th e fram e (HP 70004 display or H P 70001 mainfram e)

that contains the 70900 local oscillator module. Disconnect any

externa l computers from th is same H P-IB line, as th ere can only be

one controller on the MMS Bus at a time. Pr ess the MENU key and

then press:

Misc, more , Catalo g & MSI. The instr umen ts tr ace display

will shrink an d th e list of th e cur rent Mass Stora ge Is

assignment will be presented a s in figur e B-1.

Press the HPIB disk key for t he cat alog of the floppy disk.

Note: If you do not get a catalog of the floppy disk, check that

the number displayed after pressing HPIB DISK matches

the H P-IB address and un it num ber of the externa l disk

drive. The unit nu mber is entered with the H P-IB address,

separated by a period, with the numeric keypad. (Typically a

dua l floppy disk r eferences the left drive as 0 an d th e right

drive a s 1. A ha rd disk/floppy device t ypically r eferences

the floppy drive as 1.)

Locate th e DLP to be added to the instrument and determine

its file number.

Press the LOAD FILE key and ent er th e file num ber of th e

DLP.

The system will download t he program from t he disk into th e

instr um ent memory, if ther e is sufficient r oom, an d assign t he DLP

to a blank key on th e USER menu.


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Appen dix CRemote RIN Comman ds

This appendix contains t he remote, or programm ing, comman ds to

opera te t he t wo RIN functions of the HP 70810B light wave section.

These comman ds ar e different th an the r emote comman d for RIN

th at is supported by th e HP 70810A module. For additional infor-ma tion on synta x, par am eters, or fun ctions, refer to the H P 70900

Local Oscillator Pr ogra mming Man ua l. The H P 70900 is the

cont roller an d system ma ster for H P 71400C and 71401C systems

th at conta in the HP 70810B module.

HP 71400C and 714001C lightwave an alyzers cont aining th e

HP 70810B module ha ve two RIN functions available. The first is

available in t he ma rker menu s. The second is a downloada ble

program (DLP); see appendix B. Both functions are accessible by

remote comma nds. P rior to intr oduction of th e HP 70810B module,

th e 70810A module measu red only RINSystem with the remote

command MKRIN. Program s writt en for t hese older m odules will

need to cha nge any MKRIN comma nd used t o allow operation withth e HP 70810B module.

Following are t he RIN H P 70810B comman ds.

RIN System HP 70810B function

Menu function: RIN S YS

Remote comman d: MKRINSYS

ON

OF F

Freq units

UP

DN

For example the comman d MKRINSYS 1 GHz will place th e

mark er at 1 GHz and a ctivate RIN S YS . Following t he RIN com-

mand, an MKA? comma nd will retu rn t he value of th e mar ker,

RI NSystem . The arr ow comma nds, DN and UP, ar e also available.

Results will always be retur ned a s th e ra tio of the electrical values.

RIN Laser DLP

A downloadable program (DLP) is provided with each HP 70810B

module for determ ining RINLaser.

The DLP must be loaded into the instrum ent. Adjust the instr ument

to th e requ ired sett ings (such as frequen cy location, r esolutionband width, video band width, an d ma rker location) before the DLP

is run. The RIN? comman d will activate th e progra m, run a single

measu remen t, and r etur n th e following four values in order:

RINLaser, RINSystem, therm al noise term , and sh ot noise term .

Fu th er infromat ion ma ybe obtained from the system programm ing

manual .


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For more information on Hewlett-Packard Test and Measurementproducts, applications, or services,please cal l your local Hewlett-

Pack ard sales office. A currentlisting is a vailable via th e World-wide Web through AccessHP athttp://ww w.hp.com. If you do nothave access to the internet pleasecontact one of the HP cen ters l i stedbelow and th ey wi l l direct you toyour nearest HP representative .

United States:Hewlett-Packard CompanyTest and Measurement Organization5301 Stevens Creek Blvd.Bldg. 51L-SCSant a Clara, CA 95052-80591 800 452 4844

Canada:

Hewlett-Packard Canada Ltd.5150 Spectru m WayMississauga, OntarioL4W 5G1(905) 206 4725

Europe:Hewlett-PackardEuropean Marketing CentreP.O. Box 9991180 AZ AmstelveenThe Netherlands

Japan:Hewlett-Packard J apan Ltd.Measurement Assistance Center9-1, Takakura-Cho, Hachioji-Shi,Tokyo 192, J apa n

Tel: (81-426) 48-0722Fax: (81-426) 48-1073

Latin America:Hewlett-PackardLatin American Region Headquar ters5200 Blue La goon Dr ive, 9th F loorMiam i, Florida 33126, U.S.A.(305) 267 4245/4220

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17-21/F Sh ell Tower, Times Squ ar e,1 Matheson Str eet, Causeway Bay,Hong KongFa x: (852) 2506 9285

Data Subject to ChangeCopy right 1991, 1996Hewlett-Packard CompanyPrinte d in U.S.A. 7/965091 2196E

Documents

HP-PN71400-1_Lightwave Signal Analyzers Measure Relative Intensity Noise