Optics Communications 225 (2003) 371–376

www.elsevier.com/locate/optcom

Tunable narrowband quasi-modeless laser

I.G. Koprinkov *,1, Akira Suda, Katsumi Midorikawa

RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan

Received 18 November 2002; received in revised form 12 July 2003; accepted 19 July 2003

Abstract

A simple and reliable ‘‘modeless’’ dye laser is created. Spectrally dense, tunable generation of strongly suppressed

mode structure of about 6 GHz bandwidth is obtained. The laser parameters allow to detect simultaneously the entire

inhomogeneously broadened spectral line for a number of practical cases.

� 2003 Elsevier B.V. All rights reserved.

PACS: 42.60.By; 42.60.Da; 42.55.Mv

Keywords: Modeless laser; Dye laser; Laser cavity; Cavity modes

1. Introduction

The discrete mode/frequency structure of thelaser cavity field becomes inappropriate for some

applications. In the case of inhomogeneous

broadening of the atomic transitions, the laser

emission interacts with those atomic groups only,

whose transitions coincide with the frequencies of

the laser cavity modes. The atomic groups having

off-resonant transitions do not practically absorb

laser photons and may become lost for the ex-periment. This problem becomes crucial when the

number of the particles (atoms, molecules) to be

* Corresponding author. Tel.: +359-2-9653072; fax: +359-2-

683215.

E-mail address: igk@tu-sofia.bg (I.G. Koprinkov).1 Present address: Department of Applied Physics, Technical

University of Sofia, 8, Kliment Ochridski blvd., 1756 Sofia,

Bulgaria.

0030-4018/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/j.optcom.2003.07.036

detected is very low. That is why, creation of

tunable laser of continuum spectrum without (or

having strongly suppressed) frequency structureand a bandwidth comparable to the typical

Doppler bandwidth, is important. Lasers without

mode structure, called modeless lasers (MLLs),

were demonstrated in a number of works [1–4].

Nowadays, the MLLs are important tool in the

broadband single-shot CARS spectrometers [3,4]

and allow substantial improvement of the CARS

performance. The MLLs are actually travellingwave amplifiers working in amplified spontaneous

emission (ASE) mode, i.e., the ASE originating

from the end of the active medium is amplified

during each pass experiencing [1–3] or not [3] se-

lective spectral control. The MLLs without selec-

tive spectral control [3] or using low-pass number

amplification [1] generate too wide spectrum,

which, while suitable for broadband CARS spec-troscopy, becomes inappropriate for spectroscopy

ed.

372 I.G. Koprinkov et al. / Optics Communications 225 (2003) 371–376

of Doppler broadened atomic or molecular lines,

whose bandwidth is in the GHz range. At properly

designed MLLs, generation of tunable modeless

emission of single-shot bandwidth of about 6 GHz

is achieved [2]. This, however, is obtained at the

expense of complications in the optical scheme,thus imposing additional requirements in the laser

alignment and operation. Here, we propose an

alternative approach, which allows to create a

simple and reliable (quasi) MLL, generating nar-

rowband tunable emission. The laser we propose

actually has cavity and thus, cavity modes. How-

ever, due to the very low Q-factor used, the mode

bandwidth is wide and the adjacent modes aresubstantially overlapped so that the emission is, in

fact, continuum or quasi-modeless. To the best of

our knowledge, such an approach in the creation

of MLL is proposed for the first time [5] and its

design and properties are considered in the present

paper. The choice of the particular laser medium

and the other laser parameters was also dictated by

the intended application of such a MLL, namely,optical detection of inner shell ionized lithium at-

oms [6] using 1s2s(3S)–1s2p(3P) triplet transition at

548.5 nm.

2. Quasi-modeless cavity

The present approach of ‘‘modeless’’ laser isbased on a concept of mode overlapping. The mode

bandwidth DmMB, the free spectral range (FSR)

DmFSR, and the finesse F of the laser cavity are

related by the following expression [7,8]:

DmMB ¼ DmFSRF

; ð1Þ

Table 1

The finesse, mode bandwidth, and mode filling factor (assuming Lor

cavity consisting of combinations of total reflectance mirror (R ¼ 0:9

single Fresnel reflection wedge glass plate (R ¼ 0:04)

R1 0.99 0.99

R2 0.99 0.08

F 310 2.32

DmMB (GHz) 0.005 0.72

MFF 0.005 0.50

where the finesse is determined by the reflectivity

of the cavity mirrors R1 and R2

F ¼ pðR1R2Þ1=4

1� ðR1R2Þ1=2: ð2Þ

Consequently, if the finesse is less than unity, the

adjacent modes of a bare laser cavity must strongly

overlap each other.

For the quantitative estimation of the modeless

properties of given laser cavity, a quantity called

mode-filling factor (MFF) will be introduced. It

can be expressed as a fraction of the rectangular‘‘area’’ of mode peak intensity (gð0Þ) height, and 1

FSR (DmFSR) width, covered by two normalized (or

equal peak intensity) adjacent cavity modes gðmÞ

MFF ¼2R DmFSR=20

gðmÞdmgð0ÞDmFSR

: ð3Þ

The relevant parameters of a two-mirror bare

cavity for particular values of the mirror reflectivity

are shown in Table 1. As can be seen, finesse below

unity (F ¼ 0:65) can be achieved (based on thecavity mirrors only) using the Fresnel reflection of

two uncoated wedge glass plates as cavity mirrors.

The MFF of such a cavity approaches 0.90. The

corresponding Q-factor at k ¼ 550 nm is Q ¼ ð2L=kÞF � 2� 105, and the effective photon lifetime in

the cavity, s ¼ Qk=2pc, is 60 ps. Thus, in the

nanosecond time scale, the emission of such a laser

entirely follows the emission dynamic of the activemedium. Preliminary prescribed cavity parameters

can be obtained using dielectric mirrors of suitable

reflectivity. Approaching the limiting case R1 ! 0,

R2 ! 0, the laser is, of course, converging on the

true, ASE-MLL, whose spectral selection and

tuning requires an auxiliary optics [2].

entzian mode shape gðmÞ) of a L ¼ 9 cm long two-mirror bare

9), double Fresnel reflections parallel glass plate (R ¼ 0:08) and

0.99 0.04

0.04 0.04

1.75 0.65

0.95 2.57

0.60 0.89

Fig. 1. Optical scheme of the laser: DC, dye cell; G, diffraction

grating; M1 and M2, wedge glass plates.

I.G. Koprinkov et al. / Optics Communications 225 (2003) 371–376 373

In a real laser, other optical elements also in-

troduce losses. For the MLL considered here,Fig. 1, the main additional reduction of the effec-

tive cavity Q-factor comes from the grazing inci-

dence arrangement of the grating. That is why, the

actual finesse could be substantially smaller than

that one given in Table 1. Such extra losses are

inevitable for a selective cavity and, in the same

time, enforce the modeless properties. Overlapping

of the cavity modes, however, is not enough toobtain pronounced modeless operation because

the emission strongly narrows due to the optical

gain. That is why, besides of low cavity Q-factor,

other conditions that contribute to the modeless

operation, must be also assured. The most im-

portant of these are the relatively low pump in-

tensity (to reduce the single-pass optical gain) and

appropriate short pump pulse (to reduce thenumber of cavity round-trips). Both conditions

can be met, e.g., keeping the pump energy close to

the laser threshold. The modeless properties of the

generated laser emission can be also characterized

by the MFF.

For the experimental check of the above ap-

proach we have used a dye laser. Laser dyes,

having large stimulated emission cross-section(r � 10�16 cm2), are suitable for the present MLL

concept because high enough gain can be easily

achieved at low pump energy.

3. Experimental set-up

The scheme of the ‘‘modeless’’ laser is shown inFig. 1. For the sake of simplicity, the laser cavity is

formed by two uncoated wedge glass plates

(R1 ¼ R2 ¼ 0:04) as an output mirror M1 and a

tuning mirror M2. The laser active medium is an

ethanol solution of Rhodamin 575 (Exciton, Inc.).

The dye cell DC is 2 mm thick static cell, placed at

about 45� (or Brewster angle, if necessary) with

respect to the optical axis to avoid feedback from

the cell walls as well as to extend the length of theactive medium. Dye concentration of about

1� 10�3 M/l was found to be optimal for the

present purpose. The length of the laser cavity was

set at L � 9 cm. At these parameters, the FSR and

the bare cavity mode bandwidth (based on the

cavity mirrors, only) are DmFSR ¼ 1:67 GHz and

DmMB ¼ 2:57 GHz, respectively. The frequency

selection element is 2400 l/mm grazing incidencediffraction grating G, placed at about 88� angle ofincidence. The MLL has been pumped by the

second harmonic of a Nd:YAG laser having 11 ns

pulse duration. Longitudinal pumping has been

used to ensure better matching between the pump

and the dye laser modes. The pump emission was

focussed by a f ¼ 30 cm spherical lens. The dye

cell was placed at about 3 cm before the focalpoint. The spectrum of the laser was analyzed by a

Fabry–Perot etalon of 30 GHz FSR and finesse of

about 30. The energy of the dye laser was mea-

sured by a Rm-3700 Universal Radiometer – Laser

Probe Inc. and a RjP-735 energy probe. The pulse

duration was measured by a fast digital oscillo-

scope Tektronix DSA 602 and a PIN photodiode

S-5972.

4. Results and discussions

The mode properties of the laser spectrum have

primary importance for the present study and the

resolution of the available Fabry–Perot etalon will

be discussed first. The Fabry–Perot cavity can betreated in terms of dumping oscillator, whose fre-

quency transmission function has Lorentzian

shape of, in our case, 1 GHz mode bandwidth.

Thus, if not wider than the etalon transmission

bandwidth, two Lorentzian shaped adjacent laser

modes (1.67 GHz separation) will mutually over-

lap each other slightly below 0.25 level from their

peak intensity. The relation between the resultantintensity minimum and maximum of their super-

position is thus Imin=Imax 6 0:50. This is well below

374 I.G. Koprinkov et al. / Optics Communications 225 (2003) 371–376

the critical value of about 0.8 for the resolution of

two spectral lines [9], and two such modes must be

well resolved by the etalon. Usually, the experi-

mentally observed laser modes do not have so wide

frequency wings as the Lorentzian profile and, at

equal other conditions, Imin=Imax below 0.50 mustbe expected. Thus, if higher than 0.5 Imin=Imax is

observed, it must be attributed to the laser mode

bandwidth. In relation with the above discussion,

the following criterion of modeless emission will be

proposed: the laser emission will be considered

modeless if two adjacent cavity modes are unre-

solved according to the criterion of resolution of

two spectral lines, i.e., the condition Imin=Imax P 0:8must hold [10].

A single-shot dye laser spectrum at 0.5 mJ

pump energy is shown in Fig. 2(a). As can be seen

from the Fabry–Perot fringes, the laser mode

structure is well suppressed and practically – in-

distinguishable. The bandwidth of the generation

is about 6 GHz and consists of 3–5 well-over-

lapped cavity modes. Increasing the pump energyresults in increased spectral bandwidth and more

pronounced mode structure. Due to fluctuations,

this also happens in about 20–30% of the cases at

Fig. 2. Single-shot laser spectra at 0.5 mJ average pump energy.

For more details see the text.

0.5 mJ (average) pump energy when high-energy

pulse hits the active medium. The mode structure

may look better suppressed than it really is if the

modes have some smooth, e.g., bell-shape, peak

intensity distribution, Fig. 2(a). The actual mode

overlapping will look realistic if the modes havenearly equal peak intensity (‘‘top-flat’’ distribu-

tion). This, however, does not usually take place in

the laser operation because of the mode gain

competition and the selective cavity losses, and

may happen accidentally as a fluctuation. Such a

case is shown in Fig. 2(b), where the generated six

modes have nearly equal peak intensity. The rela-

tion between the adjacent intensity minima andmaxima in that case is about Imin=Imax � 0:9. Thisis well above the accepted 0.8 value for the mode

resolution and represents an indication of good

mode overlapping. The tuning range of the dye

laser at 0.5 mJ pump energy is 544–554 nm.

The dependence of the pulse energy of the MLL

(M1 output) vs pump energy and the correspond-

ing energy conversion efficiency are shown inFig. 3. Despite the very high cavity losses, the

MLL does not require high pump energy. Stable

laser operation can be achieved, e.g., at about 150

lJ pump energy. Pulse energy of about 5 lJ is

obtained at 1 mJ pump energy. This determines

the conversion efficiency of about 0.5%, based on

the M1 output only. As must be expected, the en-

ergy conversion efficiency of such an oscillator isnot high because of the very low cavity feedback

and the high losses of the grazing incidence grat-

ing. Within the energy range studied, the energy

conversion efficiency increases when the pump

0

2

4

6

0

0.2

0.4

0.6

0 200 400 600 800 1000

Dye

lase

ren

ergy

[µJ]

Dye

lase

ref

fici

ency

[%]

dye laser energy

dye laser efficiency

Fig. 3. Laser energy and energy conversion efficiency vs pump

energy relationships.

I.G. Koprinkov et al. / Optics Communications 225 (2003) 371–376 375

energy increases, Fig. 3. This, however, also in-

creases the spectral bandwidth and the pulse du-

ration. The oscillator output can be amplified by a

standard amplifier if higher energy is required for

particular application. In general, better efficiency

can be expected using Rhodamin 6G dye. The low-energy pump regime, used here, ensures long life-

time of the laser dye even using a static dye cell.

The pulse duration of the laser, measured at

0.35 mJ pump energy, was 2 ns. Thus, the laser

emission above the threshold is formed for about

three cavity round-trips. The peak power is 500 W

and the power conversion efficiency is 1.5%. The

pulse duration gradually increases when the pumpenergy increases, Fig. 4. The oscilloscope traces

show that this results mainly from the extension of

the trailing edge of the pulse. The later tends to

transform into a second pulse at pump energy

above 1 mJ. The second pulse becomes well ex-

pressed if replacing the glass plate M2 by a high-

reflectivity (aluminum) mirror. Thus, at low pump

energy/feedback, the trailing part of the pumppulse is not effectively converted in laser genera-

tion. This shows that the energy conversion effi-

ciency can be improved using shorter pump pulses.

The general advantage of the present approach

of MLL is that a tunable, narrowband, spectrally

dense emission is generated by means of simple

laser scheme, while still preserving the cavity

control on the laser generation. The alignment ofthis laser is as simple as the other similar grazing

incidence dye lasers. In the lack of experience,

lasing can be initially achieved using an auxiliary

total reflectance mirror placed behind the glass

plate M2. Then, the glass plate M2 is aligning so as

Fig. 4. Laser pulse duration vs pump energy.

to get generation by means of M1–M2 cavity, while

the auxiliary mirror is misaligning and removing

from the laser.

The MLL described here was particularly de-

signed for optical detection of inner shell lithium

ions using 1s2s(3S)–1s2p(3P) triplet transitionat 548.5 nm. The transition bandwidth at high-

density lithium vapors (about 1017 cm�3 at 900 �Cvapor temperature) and low degree of ionization

(Stark broadening due to the electric field of the

ions [6] can thus be neglected) is dominated by the

Doppler broadening and is estimated to be about 5

GHz. The lifetime of 1s2s(3S) (radiatively meta-

stable) and 1s2p(3P) states is estimated to be about20 and 15 ns, respectively, based on the collisions

with neutrals and spontaneous emission (3P state).

Thus, the MLL parameters (tuning range, band-

width, and pulse duration) well satisfy the re-

quirements for optical detection of 1s2s(3S) lithium

ions. Of course, the MLL described here is also

suitable for other spectroscopic applications when

tunable and spectrally dense emission over theDoppler bandwidth of given transition is required.

In conclusion, a simple and reliable ‘‘modeless’’

dye laser has been created. Spectrally dense, tun-

able generation of about 6 GHz bandwidth is

obtained. The low Q-factor and the low-energy

pump regime help to generate a quasi-modeless

emission of short pulse duration, and long lifetime

of the laser dye. The laser parameters allow todetect simultaneously the entire inhomogeneously

broadened spectral line for a number of practical

cases.

Acknowledgements

The authors are grateful to Dr. Tohru Kobay-ashi for the loan of technical equipment.

References

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mode bandwidth can be deduced more precisely. How-

ever, reducing the mirror reflectivity down to the range

of interest results in minima of the transmission function,

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as the full width at half maximum. That is why, our

conclusions will be based on Eq. (1), assuming modes of

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[10] It is, of course, understood that the transmission function

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the laser modes.