External Cavity Diode Laser

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910 OPTICS LETTERS / Vol. 16, No. 12 / June 15, 1991

External-cavity diode laser using a grazing-incidence diffractiongrating

K. C. Harvey and C. J. Myatt

Department of Physics, Dedman College, Southern Methodist University, Dallas,Texas 75275

Received January 14, 1991

An external cavity has been developed for use with commercial diode lasers. It uses a diffraction grating at grazing

incidence for wavelength selection and output coupling. This configuration allows a GaAlAs diode laser to be tuned

conveniently anywhere in a range greater than 20 nm. Also, the linewidth is reduced by a factor of more than 1000

from 40 MHz to less than 10 kHz. The new laser system should be useful for high-resolution spectroscopy, laser

cooling of atoms, time standards, and coherent optical communications.

For some applications, such as spectroscopy and lasercooling of atoms, a tunable single-frequency laser isrequired that is versatile and inexpensive. It needs tohave a narrow linewidth but only modest power. Asolitary diode laser largely satisfies this need. It canbe tuned over a limited range by changing the tem-perature or current of the diode. However, the tuningrange may have gaps in it, and the linewidth is large fora cw laser.' Further, a solitary diode laser is sensitiveto optical feedback. However, if an external cavity iscoupled to the diode laser, the linewidth is narrowedand the frequency tuning of the diode laser is im-

proved.A diode laser has been weakly coupled to an externalcavity that consists of a simple external mirror,2 andan 6talon has been added to control the mode selec-tion.3 More recently a laser diode has been opticallylocked to an external high-finesse cavity.4 This pro-duced a greatly narrowed linewidth of only 20 kHz.The wavelength of the diode laser was stabilized by thecavity, and a commercial diode could be used. How-ever, this approach is somewhat complex. Also, sinceit is restricted to the weak-feedback regime, the wave-length is limited to tuning ranges near the axial-moderesonances of the laser diode. Therefore there may

still be gaps in the tuning range.The diode laser has also been strongly coupled to an

external cavity that has a frequency-selective ele-ment.5 An external cavity with a grating in Littrowmount produced a linewidth of 10 kHz.6'7 Thislinewidth was later considerably narrowed to as low as300 Hz by appropriate detuning of the feedback phaseof the external cavity from resonance with the fieldreflected from the antireflection-coated facet of thediode laser.8 However, these techniques require a di-ode laser to have specially prepared antireflectioncoatings on the facets. For simplicity, it is highlydesirable to use commercial diode lasers. Configura-

tions with diffraction gratings in Littrow mount havebeen used with commercial diode lasers.9"10 However,in these types of cavity the laser beam will rotate andtranslate when the frequency is changed by movingthe grating.

We describe here a new external cavity that is usedwith a commercial GaAlAs diode laser. It is simpleand versatile and is made of standard optical compo-nents. A diffraction grating at grazing incidenceserves for wavelength selection and output coupling.The laser is single frequency, has a narrow linewidth,and is continuously tunable over a wide range. Thelaser beam has good directional stability when it istuned. The laser cavity is shown in Fig. 1. It is athree-mirror cavity" that consists of a high-reflection-coated rear facet of the diode laser, the lasing mediumof the .diode, the antireflection-coated front facet of

the diode laser, the window of the diode-laser case, acollimating lens, a diffraction grating at grazing inci-dence, and an external mirror. The zeroth-order re-flection from the grating is the output of the laser.The first-order reflection from the grating is reflectedback into the laser by the external mirror. One end ofthe laser cavity is the rear facet of the diode laser, andthe other end is the external mirror. The frequency istuned by moving the external mirror. This arrange-ment is similar to the Littman configuration1 2-'4 forpulsed lasers. However, it differs in that here a lens isadded to collimate the beam, the gain medium is in awaveguide, and the laser is cw.

,A tuningPZT

zN \ mirtror

output beam

diode

loser

diffraction

LI1grat I ng

Fig. 1. Schematic diagram of the laser. PZT, piezoelectricceramic.

0146-9592/91/120910-03$5.00/0 © 1991 Optical Society of America



June 15, 1991 / Vol. 16, No. 12 / OPTICS LETTERS 911

The coarse tuning of the cavity is determined by thegrating equation,'5

XL= m(sin a + sin), (1)

where XL is the wavelength of the laser, d is the gratingspacing, and m is the diffraction order. a and fiarethe angles shown in Fig. 1. For grazing incidence,sin a - 1. The single-pass dispersion of the grating isintrinsically twice as great as cavities that use a gratingin Littrow mount. The single-pass linewidth(FWHM) of the cavity due to the grating is

dw,AXG = cos a, (2)

where w, is the beam waist at the diode and f is thefocal length of the collimating lens. Cavities that usea diffraction grating in Littrow mount typically have a

larger grating passband because fewer lines of the dif-fraction grating are covered by the laser beam. Theincreased dispersion of the grazing-incidence diffrac-tion grating improves selection of axial cavity modesand biasing of external cavity modes to diode-lasercavity modes, which is useful for some linewidth-re-duction techniques.8 The linewidth of the laser isgiven by the modified Schawlow-Townes formula.5' 7

The addition of an external cavity to a solitary diodelaser reduces the linewidth primarily because the cavi-ty length is greatly increased, which reduces the pas-sive cavity linewidth and the relative influence of fluc-tuations in the index of refraction of the gain medi-um.5

The external-cavity laser was constructed with acommercial GaAlAs diode laser (Sharp LTO24MD)that lased at a wavelength of 780 nm at room tempera-ture. The output facet had an antireflection coatingthat had a reflectivity estimated to be between 2% and10%. The other facet had a high-reflection coating ofapproximately 97% reflectivity. The solitary diodelaser had a free-running linewidth of roughly 40 MHzand an axial mode spacing of 0.31 nm. The tempera-ture of the diode laser was held constant by a thermo-electric cooler and a controller unit. The temperaturewas typically 190C. The diode current was suppliedby a precision current source. The diode current wastypically 90 mA. The output of the diode laser wascollimated by an antireflection-coated, bi-aspheric plastic lens that had a numerical aperture of0.45 and a focal length of 4.5 mm. An 1800-line/mmholographic diffraction grating (American Holograph-ic, catalog no. 135.1800)was used in grazing incidence.The diffraction angle a was measured to be approxi-mately 85°. The diffraction efficiency into the firstorder was measured to be 40%. Thus, neglecting cou-pling losses, 15% of the power was returned to thediode-laser amplifer. The internal cavity modes ofthe diode laser were therefore strongly coupled to theexternal cavity. The junction plane of the diode laser

was oriented to be perpendicular to the grooves of thediffraction grating to optimize the efficiency of thegrating. For the parameters of this cavity, Eq. (2)gives the passband of the diffraction grating to be A;\G

= 0.03 nm or,AVG= 14 GHz. The external mirror wascoated for >99% reflection in the near infrared andwas translated by a piezoelectric ceramic. The aver-age length of the laser cavity was roughly 12 cm. Thiscorresponds to an axial mode spacing of 1.2 GHz. Thecomponents of the laser were rigidly mounted on analuminum plate. The laser was enclosed in a clear

Lucite box to reduce air currents and stabilize thetemperature of the laser cavity. This box was sur-rounded by another box made of polystyrene foamboard to insulate the laser cavity and reduce tempera-ture drift. The laser and associated optics were on aoptical table that was pneumatically floated on fourlegs to provide vibration isolation. A charge-coupled-device television camera allowed us to observe thelaser radiation for cavity alignment and beam adjust-ments.

The coarse wavelength of the diode laser was mea-sured with a 0.5-m spectrometer. The spectrometerhad a self-scanned linear photodiode array of 512 ele-ments for real-time recording. The tuning range ofthe laser is shown in Fig. 2. For this figure the fre-quency was changed by rotating the external mirrorwhile leaving the diode temperature and current con-stant. The laser could be tuned within 20 rim aboutits doped wavelength. If the temperature of the diodelaser were changed it is expected that the tuning rangewould increase to 30 nm. To observe the wavelengthwith higher resolution, we used a scanning confocalinterferometer that had a free spectral range of 8 GHzand a finesse of 200. The laser could be continuouslytuned 6 GHz by translating the external mirror withthe piezoelectric transducer. When the laser frequen-cy was scanned, the diode current or temperature did

not have to be changed but instead was held constant.For additional diagnostics, we could observe the reso-nance fluorescence from an atomic rubidium vaporcell. In general, to tune the laser to a particular wave-length, the external cavity was first misaligned and thediode current was adjusted to locate the wavelength of

20 r

U,

3:

E- 15

HE

a

00.

10 L

-15

.

-10 -5 0 5

wavelength ( nm )

10 15

Fig. 2. Tuning curve of the laser. The point under thecurve is the power of the laser when the external cavity ismisaligned and the diode lases from its front facet.



912 OPTICS LETTERS / Vol. 16, No. 12 / June 15, 1991

Fig. 3. Beat frequency for the lasers. The intrinsiclinewidth of the laser is less than 10 kHz. The horizontalaxis is the frequency; it is linear, and each division is 1.0MHz. The vertical axis is the signal intensity, and it islogarithmic. Center frequency, 54 MHz; resolution band-width, 10 kHz; scan time, 2.06 s.

6

5

?4 - 1

C21

0 I-

0.0 0.5 1.0 1.5 2.0frequency ( GHz )

2.5 3.0

Fig. 4. Saturated absorption spectrum of 85Rb and 87 Rb.The peaks on the right are the 5SI1 2(F = 3) to 5P3/transi-tions for 85Rb, and the peaks on the left are the 5S,12 (F = 2)to 5P3 / 2 transitions for 8 7Rb.

the solitary diode laser near the region of interest, thenthe external cavity was adjusted to obtain lasing offthe grating and external mirror.

The linewidth and frequency stability were mea-

sured with a heterodyne technique. Two identicalexternal-cavity diode lasers were constructed. Thebeams from the two identical lasers were mixed in afast P-I-N diode. The signal from the photodiode wasthen amplified and sent to a spectrum analyzer. Theheterodyne spectrum is shown in Fig. 3. For longersweeps a broad pedestal with a width of 200 kHz wasobserved. This pedestal was primarily caused bytechnical noise such as mechanical vibrations, noise inthe diode current, optical feedback, and residual indexof refraction noise. No optical isolators were used inthese measurements. It was observed qualitativelythat the diode laser with the external cavity was much

less sensitive than the solitary diode laser to randombackscattering or reflections from optical compo-nents. However, the effect of feedback from the con-focal interferometer was observed, and the confocalinterferometer was blocked during the heterodyne

measurements. There was a small dependence of thelaser frequency on the diode current of 16 MHz/mA,but this is much smaller than for a solitary diode laser.This effect provides a convenient way to tune or stabi-lize the laser over small frequency intervals.

To demonstrate the usefulness of this new externalcavity we have used it as a laser spectrometer systemto observe the Doppler-free, saturated-absorptionspectrum of85Rb and 87Rb. A typical spectrum of thetransition from 5S1,2 to 5P3/2 is shown in Fig. 4.

For some applications this external-cavity configu-ration has definite advantages. It uses a commercialdiode laser that has no special coatings or other modi-fications. It has better immunity from optical feed-back than does a solitary diode laser. It has a muchsmaller laser linewidth compared with the solitary di-ode laser. It has the possibility of a large, continuoustuning range.13'6 The laser beam has good directionalstability since the direction of the beam is determinedby the waveguide of the diode laser and the diffractiongrating, which do not move when the frequency istuned. Although the long-term frequency stability ofthis laser is less than that in Ref.4, it can be improvedby locking the laser to a passive external cavity. Thislaser should be useful for a number of applications,including high-resolution spectroscopy, the trappingand cooling of atoms, coherent optical communica-tions, and frequency standards.

We thank W. Johnston for making parts of the ap-paratus and A. K. Hemann for building the powersupply for the photodiode array.

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