2586 OPTICS LETTERS / Vol. 31, No. 17 / September 1, 2006

Tunable single-mode Fabry–Perot laser diode usinga built-in external cavity and its modulation

characteristics

Yong Deok Jeong and Yong Hyub WonOptical Internet Research Center, Information and Communications University, Yuseong-Gu,

Daejeon, 305-732, Korea

Sang Ook Choi and Jong Hyun YoonNeowave, Inc., Gwanyang-dong, Dongan-gu, Anyang, 431-712, Korea

Received May 8, 2006; revised June 7, 2006; accepted June 8, 2006;posted June 12, 2006 (Doc. ID 70709); published August 9, 2006

A tunable single-mode laser is obtained by using a weakly coupled cavity structure involved in a coaxiallypackaged Fabry–Perot laser diode. The cleaved end facet of the coupling fiber becomes an optical reflectorand forms an external cavity with a laser facet. The single-mode oscillation condition is controlled and sta-bilized by tuning the operating temperature. The tuning range is about 10 nm with the side-mode suppres-sion ratio of more than 27 dB when the temperature changes from 11.5°C to 25°C. Direct modulation char-acteristics were investigated, and our results show that a shorter external cavity can bear deepermodulation depth. © 2006 Optical Society of America

OCIS codes: 140.2020, 140.3570, 140.3600, 140.5960.

A tunable laser offers greater flexibility and trans-parency in the wavelength division multiplexing(WDM) optical network, and reduces the inventorycost of optical transmitters. Tunable laser technolo-gies are mainly divided into monolithic integrated la-sers (MILs) and external cavity lasers (ECLs).1 MILs,including distributed Bragg reflector laser with grat-ing tuning section or selectable distributed feedbacklaser array, have advantages of small size and fasttuning speed. Recently, ECLs have been subject to in-tensive attention because of their good spectral prop-erties and wide tuning range. The structure and tun-ing mechanism of ECLs are determined by the classof mode selection filter, such as tunable filter,2–4 mi-croelectromechanical systems grating,5 or fiber Bragggrating,6 which is tuned by electrical, thermal, or me-chanical control. Another method to construct anECL uses optical feedback from a simple reflectorthat forms an external cavity with a laser diode facet.This method has been widely investigated for variousexternal cavity lengths from several tens of microme-ters to a few centimeters.7–9 Although these schemesshow efficient performance, they require very precisepiezoelectric transducer control and unique packag-ing structures, which result in higher cost. Moreover,many past studies have focused only on spectral be-havior and overlooked direct modulation characteris-tics. In this Letter we propose a simple tunable laserusing a Fabry–Perot laser diode (FP-LD) employing abuilt-in external cavity, and investigate its directmodulation characteristics with varying externalcavity lengths. The facet of the coupling fiber be-comes an optical partial reflector and forms an exter-nal cavity with a laser facet. The external cavitylength is a few millimeters. We use a low-cost tran-sistor outlook–Can (TO–CAN)-type package with co-axial housing that is a well-developed technology for

mass production. The only additional component is a

0146-9592/06/172586-3/$15.00 ©

thermoelectric cooler (TEC) module that is used totune the single-mode oscillation condition. Neverthe-less, it is worth noting that any tunable laser diodeadopts a temperature controller to stabilize its lasingwavelength.

The cleaved end facet of the fiber provides approxi-mately 4% Fresnel reflection. The reflected lightcouples with the lasing modes within the laser cavity,and this coupling can induce optical feedbacknoise.10,11 For this reason, in the commercial pig-tailed FP-LD, the facet of the coupling fiber generallyhas an inclination of 6°–8° in order to prevent opticalfeedback. However, optical feedback does not alwaysinduce noise. Proper mode matching gives linewidthreduction for the single-mode laser or strong-sidemode suppression for the multimode laser.12–14 Themode-matching condition is dominated by feedbacklevel and the relationship between the external cav-ity length, Lext, and the laser cavity length, LL. Thuswe purposely eliminated an inclination from the cou-pling fiber and formed an external cavity between thelaser diode and the fiber. By adjusting the operatingtemperature using a TEC module, the mode-matching condition for the two mode combs of bothcavities can be controlled to achieve single-mode os-cillation. The mode selection mechanism has been ex-plained by Rang and Kobayashi.12 Since the modespacing of the external cavity (��ext=�2 /2nextLext,where next is the refractive index of the external cav-ity) is several times shorter than that of the lasercavity (��L=�2 /2nLLL, where nL is the refractive in-dex of the laser cavity), single-mode selection withinthe gain bandwidth of the solitary laser diode is pos-sible only when Lext is not close to an integer multipleof LL. On the other hand, the dense mode spacing ofthe external cavity shows that a small amount of la-ser cavity length variance can rearrange the mode-

matching condition. In our experiment, the operating

2006 Optical Society of America

September 1, 2006 / Vol. 31, No. 17 / OPTICS LETTERS 2587

temperature is controlled to adjust laser cavitylength. As the temperature of the laser diodechanges, the refractive index of the active regionchanges. This shift leads to a change of optical pathlength in the laser diode, so that the optimal mode-matching condition for a single-mode oscillation canbe obtained at a specific temperature.

Figure 1(a) shows the cross section of the commer-cial coaxially packaged FP-LD, and the overall struc-ture is not changed in our experiment. The appear-ance of the packaged tunable LD with attached TECmodule is shown in Fig. 1(b). The FP-LD used in theexperiment is the Mitsubishi ML925BF TO–CANthat has an InGaAsP multiple quantum well struc-ture with 300 �m cavity length, and the cleaved facetreflectances at both sides of the FP-LD are about30%. The light from the laser diode is coupled intothe optical fiber via an aspherical glass lens that isantireflection coated for 1550 nm. Figure 2 shows theoutput optical spectra of (a) the conventional and (b)the proposed FP-LDs. In Fig. 2(b), the largest peakhas a power of 0 dBm with 31 dB side-mode suppres-sion ratio (SMSR), while the SMSR of the conven-tional FP-LD is less than 3 dB as shown in Fig. 2(a).Since the single mode condition is affected by boththe bias current and the operating temperature, wefixed the bias current and varied the temperature byusing the external TEC. The output optical spectra ofthe proposed FP-LD in single-mode oscillation at sev-eral operating temperatures are shown in Fig. 3. Inthe measured spectrum, the temperature was ad-justed to match the single-mode oscillation condition.When the temperature changes from the locallymaximum point of SMSR, single-mode oscillation col-

Fig. 1. (a) Cross section of a coaxially packaged FP-LDthat includes a built-in external cavity. (b) Appearance ofthe packaged tunable LD with attached TEC module.

Fig. 2. Optical spectra of (a) conventional FP-LD and (b)proposed FP-LD.

lapses. If we keep on changing the temperature, how-

ever, the FP-LD again reaches single-mode oscilla-tion at an adjacent mode. The wavelength differencein comparison with the previous one is the same asthe mode spacing of the laser cavity. The peak wave-length hops successively as the temperature changesuntil the wavelength goes far from the gain band-width of the laser diode. The wavelength then jumpsto another, lower mode within the gain bandwidth.Because the gain bandwidth of the FP-LD shifts to alonger wavelength as the operating temperature in-creases, the tuning range can be extended.

Detailed tuning characteristics of the proposed la-ser diode are illustrated in Fig. 4 when the tempera-ture changes from 11.5°C to 25°C. Each mode jumprequires a temperature change of �0.5°. In otherwords, the temperature stabilization level for single-mode oscillation should be within less than 0.5°C,but it is not difficult to realize for practical use be-cause commercial TEC modules and TEC driversusually guarantee stability of less than 0.1°C. Even-tually, we achieve complete reliability when the envi-ronmental temperature changes from −10°C to+50°C. The wavelength tuning range is about 10 nm,and each wavelength has a SMSR of more than27 dB. From the starting point (a), the peak wave-length jumps to the next modes (b, c, d) successivelyuntil the wavelength reaches point (e), and then thewavelength jumps down to point a�. The wavelengthhopping then repeats as the temperature increasesuntil the wavelength gets out of the gain bandwidth.The difference between the wavelengths at b and a�is 0.36 nm, which should be equal to the longitudinalmode spacing of the external cavity. To clarify theuniform performances for the overall tuning range,

Fig. 3. Optical spectra of proposed FP-LD in single-modeoscillation at various operating temperatures.

Fig. 4. Wavelength and SMSR as functions of

temperature.

2588 OPTICS LETTERS / Vol. 31, No. 17 / September 1, 2006

the relative intensity noises (RINs) were measuredfor rf spectral range from 3 MHz to 2.5 GHz with sev-eral tuned wavelengths. Their maximum RIN valuesare less than −140 dB/Hz over all wavelengths asshown in Fig. 5.

When the laser is modulated directly, the refractiveindex of the laser cavity changes with the modulatedcurrent. The mode-matching condition will then beperturbed at every instance. If the degree of currentmodulation exceeds a threshold level, the single-mode oscillation collapses and mode-hopping noiseincreases. The dependence of the modulation depththreshold on the external cavity length is known tobe

mth = �n�0/4LextJ���J/�n�,

where �0 is the operating wavelength, n is the refrac-tive index of the laser diode, and J is the injectedcurrent.13 This equation was investigated experimen-tally by measuring the bit error rate (BER) curves atvarious external cavity lengths as shown in Fig. 6(a).The FP-LD was directly modulated by a 155 Mbit/spattern generator with 231-1 pseudorandom non-return-to-zero signal and the output light passedthrough the bandpass filter with 3 dB bandwidth of0.4 nm. Figure 6(a) proves that the maximum extinc-tion ratio (ER) to achieve error-free operation in-creases as the external cavity length decreases. Forthe 4 mm external cavity, error-free operation wasachieved when the ER is larger than 9 dB while anerror floor appeared when the ER was only 4.11 dBfor the 8 mm external cavity. The higher bit rate lim-its the maximum ER for error-free operation because

Fig. 5. RIN as function of wavelength.

Fig. 6. BER curves with (a) various external cavitylengths at 155 Mbit/s and (b) various wavelengths at622 Mbit/s and 1.25 Gbit/s.

of the single-mode buildup time after the lasing modeshrinks to zero level. The maximum ERs at622 Mbit/s and 1.25 Gbit/s with 4 mm external cav-ity were �8 and 6 dB, respectively, and their BERperformances are shown in Fig. 6(b). The large powerpenalty at 1.25 Gbit/s compared with 622 Mbit/s wascaused mainly by the difference of receiver sensitiv-ity. The receiver for 1.25 Gbit/s in the experimenthas a sensitivity of approximately −20 dBm at 10−10

BER, while the receiver for 622 Mbit/s has a sensi-tivity of −30 dBm. Also, Fig. 6(b) indicates the similarBER performances of several tuned wavelengths.The deviation levels of four BER curves for each bitrate are less than 0.5 dB.

In conclusion, a simple and cost-effective tunablelaser is proposed and experimentally demonstrated.The overall tuning range is about 10 nm with sidemode suppression higher than 27 dB. Moreover, weinvestigated the modulation characteristics of theproposed tunable laser and proved that a shorter ex-ternal cavity length offers deeper modulation depth.The result shows that the proposed tunable laser di-ode would be a low-cost source of optical transmittersin WDM networks and, especially, optical access net-works.

This work was supported in part by the KOSEFthrough the OIRC project. The authors thank DeokJin Kim of Teradian, Inc., for his work on laser diodepackaging and RIN measurement.

Y. D. Jeong’s e-mail address is kumdong@icu.ac.kr.

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