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