tion distributed feedback laser diode (TDA-DFB-LD) is proposed and fabricated. The proposed structure provides synchronous change between the longitudinal ...
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 1, JANUARY 1998
A Tunable Distributed Amplification DFB Laser Diode (TDA-DFB-LD) Hiroyuki Ishii, Yasuhiro Kondo, Fumiyoshi Kano, and Yuzo Yoshikuni
Abstract— A novel wavelength-tunable distributed amplification distributed feedback laser diode (TDA-DFB-LD) is proposed and fabricated. The proposed structure provides synchronous change between the longitudinal mode and the Bragg wavelength in tuning; thereby continuous wavelength tuning can be realized with single control-current. Moreover, extended tuning is also possible with two control-currents. A continuous tuning range of 2.9 nm and an extended discrete tuning range of 15 nm are experimentally obtained with a 1.55-m TDA-DFB-LD. Index Terms—Distributed-feedback lasers, longitudinal modes, semiconductor lasers, tunable lasers. (a)
I. INTRODUCTION
W
AVELENGTH-TUNABLE laser diodes are promising light sources for future re-configurable wavelengthdivision-multiplexing (WDM) networks and optical measurement applications. Recently, several kinds of broadly tunable lasers have been developed, and their tuning ranges have exceeded 30 nm even under the limitation of the maximum change in the refractive index [1]–[5]. One problem of these lasers is the complexity in the tuning scheme. They require three control-currents, two for controlling the two wavelengthselective reflectors and one for controlling the longitudinal mode, in order to continuously cover a tuning range. Because the multielectrode control needs many current sources in system applications and it takes a lot of time to test the devices, reduction of the control port is important for its use in practical applications. Although we proposed a tunable-interdigitalelectrode (TIE) DBR laser to obtain single control-current tuning [6], the tuning range is restricted by the maximum change in the refractive index. So, in this letter, we propose a novel tunable distributed amplification DFB laser diode (TDADFB-LD) which is a further improvement of the TIE-DBR laser. The unique distributed amplification structure can eliminate the longitudinal mode control, and continuous tuning with a single control-current can be obtained. Moreover, extended tuning with two-control currents can be also obtained with the two-section structure. The expected tuning characteristics are presented with a fabricated device. II. STRUCTURE
AND
PRINCIPLE
The basic structure of the proposed TDA-DFB-LD is shown in Fig. 1(a). The unique structure is that optical amplification Manuscript received July 7, 1997; revised October 7, 1997. The authors are with the NTT Opto-electronics Laboratories, Atsugi-shi, Kanagawa 243-01, Japan. Publisher Item Identifier S 1041-1135(98)00476-5.
(b)
(c) Fig. 1. (a) Basic structure of TDA-DFB-LD. (b) Behavior of reflectivity and longitudinal modes. (c) Structure of two-section TDA-DFB-LD.
regions and refractive-index-control regions are distributed alternately throughout the entire laser cavity. The diffraction grating is formed on the waveguide for single-mode operation. The p-side electrodes in each region are connected to each other to form interdigital electrodes on the laser chip. When the refractive index of all the tuning regions is changed by current injection through the interdigital electrode, both the Bragg wavelength and the longitudinal-mode wavelength shift to the shorter wavelength. Because the tuning regions are distributed in the laser cavity, the Bragg wavelength is changed
1041–1135/98$10.00 1998 IEEE
ISHII et al.: A TUNABLE DISTRIBUTED AMPLIFICATION DFB LASER DIODE
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Fig. 3. Extended tuning characteristics with two control-ports.
Fig. 2. Wavelength, fiber output, and SMSR under single control-current tuning.
synchronously with the longitudinal cavity mode, as shown in Fig. 1(b). The wavelength change of the Bragg wavelength and the longitudinal mode are expressed as (1) is the change in the equivalent refractive index. where Because of the partial current injection, the wavelength change which is the longitudinal is reduced by confinement factor for the tuning sections. When the ratio of the tuning region to the active region is 1:1 ( 0.5), both the Bragg wavelength change and the longitudinal mode change vary by half of the refractive index change. Thus, the distributed amplification structure can realize continuous tuning without mode-hopping, similar to a tunable-twin-guide (TTG) laser [7] and a TIE-DBR laser [6]. A merit of the TDA-DFB-LD is that a conventional embedding re-growth technique can be used for the current confinement because of the single waveguide structure. Although the continuous tuning range is limited by the maximum change in the refractive index according to (1), extended tuning can be realized by introducing a two-section structure with modulated grating, that is a broadly tunable two section laser with a distributed amplification structure, as shown in Fig. 1(c). The laser has an interdigital electrode to inject current into all the active regions, and has two interdigital electrodes, by which tuning currents to the two sections are controlled independently. Diffraction gratings are formed partially and periodically on the tuning regions to form the sampling structure [1]. This sampled grating produces multiple reflection maxima in the reflection spectrum. The
unit length of the alternate structure, which is the sampling periods of the sampled grating, in one section is different from that in the other section. Therefore, single-mode lasing occurs at the wavelength where the reflection maximum of the one section is aligned with that of the other section. By controlling currents to the two tuning sections independently, extended tuning based on the principle of a vernier scale can be obtained, similar to a sampled grating DBR-LD [1] and a superstructuregrating DBR-LD [2]. However, the two-section TDA-DFB-LD requires only two control-currents since the longitudinal mode control is eliminated. III. EXPERIMENTS 1.55- m two-section TDA-DFB-LD’s were fabricated on InP substrate using a MOVPE growth technique. An InGaAsP ( 1.4 m) bulk layer was used for the passive tuning regions, and a strained ( 1%) multiple-quantum-well (MQW) layer ( 1.57 m) was used for the active regions. They were butt-jointed using a selective re-growth technique. The laser consisted of two sections of different unit lengths. The length of the each section was 600 mm. The unit length in the front section was 66 m, and that in the rear was 75 m. The ratio of tuning region length to the active region length was 1:2. The sampling period of the grating was the same as the unit length, and the ratio of grating length to the unit length was 0.24. The coupling coefficient of the grating was 150 cm . 1.5- m-width laser stripes were formed by -RIE, and were buried by p-n current blocking layer [8]. Both end-facets were coated with antireflection films to avoid lasing with Fabry–Perot modes. The devices were tested under continuous wave operation at 25 C. The threshold current was 40 mA without wavelength tuning. The lasing wavelength, the output power coupled to a single-mode fiber (SMF) and the side-mode suppression ratio (SMSR) are plotted in Fig. 2 as a function of the tuning current , where the two tuning electrodes are shorted. The lasing wavelength changes continuously without mode-
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 1, JANUARY 1998
in the tuning is due to the Bragg mode spacing, which is determined by the sampling period of the grating. The lasing spectra at each wavelength are shown in Fig. 4. Single-mode operation with sufficient mode suppression is obtained at all wavelengths. Several stop-bands, due to the sampled grating, are observed in the amplified spontaneous emission spectra. IV. SUMMARY A tunable distributed amplification DFB laser diode (TDADFB-LD) is proposed to improve the wavelength controllability. two-section TDA-DFB-LD’s were fabricated using a butt-joint re-growth technique. A single current continuous wavelength tuning range of 2.9 nm and an extended tuning range of 15 nm with two control currents were experimentally obtained together with high single mode stability. Fig. 4. Lasing spectra at four extended Bragg modes.
hopping. High SMSR of more than 35 dB are maintained during 2.9-nm tuning. This result indicates that a simultaneous change between the Bragg wavelength and the longitudinal mode is realized due to the distributed amplification structure. The wide tuning range of 2.9 nm is a little larger than the expected range, which is determined by (1). It can be considered that there is an effect of increase of threshold carrier density in the active regions due to the loss change in the tuning regions. The output power is relatively low compared with that of a conventional DBR laser. This is due to scattering loss at the active/passive interfaces. A coupling efficiency of more than 90% per interface is estimated from the threshold current change in comparison with a Fabry–Perot laser without interfaces. However, while the TDA-DFB-LD has essentially many active/passive interfaces, the laser reported here has 33 interfaces; consequently, this degrades the external efficiency. The coupling efficiency should be nearly 100% with careful design of the waveguide structure. The output power increases as the tuning current increases. The reason is that a part of the tuning current leaks to the active regions. Fig. 3 shows the extended tuning characteristics when the current into the two tuning electrodes is controlled, either by injecting it into the front or the rear tuning section. The lasing wavelength changed discretely at four extended Bragg modes, and a 15nm tuning range was obtained. About a 5-nm discrete change
ACKNOWLEDGMENT The authors wish to thank J. Yoshida, Y. Hasumi, T. Tamamura, and M. Yamamoto for their support and encouragement. REFERENCES [1] V. Jayaraman, Z. M. Chuang, and L. A. Coldren, “Theory, design, and performance of extended tuning range semiconductor lasers with sampled gratings,” IEEE J. Quantum. Electron., vol. 29, pp. 1824–1834, 1993. [2] Y. Tohmori, Y. Yoshikuni, H. Ishii, F. Kano, T. Tamamura, Y. Kondo, and M. Yamamoto, “Broad-range wavelength-tunable superstructure grating (SSG) DBR lasers,” IEEE J. Quantum Electron., vol. 29, pp. 1817–1823, 1993. ¨ [3] M. Oberg, P.-J. Regole, S. Nilsson, T. Klinga, L. B¨ackbom, K. Streubel, J. Wallin, and T. Kjellberg, “Complete single mode wavelength coverage over 40 nm with a super structure grating DBR laser,” IEEE J. Lightwave Technol., vol. 13, pp. 1892–1898, 1995. [4] P.-J. Rigole, S. Nilsson, L. B¨ackbom, T. Klinga, J. Wallin, B. Stalnacke, E. Berglind, and B Stoltz, “114-nm wavelength tuning range of a vertical grating assisted codirectional coupler laser with a super structure grating distributed Bragg reflector,” IEEE Photon. Technol. Lett., vol. 7, pp. 697–699, 1995. [5] H. Ishii, H. Tanobe, F. Kano, Y. Tohmori, Y. Kondo, and Y. Yoshikuni, “Broad-range wavelength coverage (62.4 nm) with superstructuregrating DBR laser,” Electron. Lett., vol. 32, pp. 454–455, 1996. [6] H. Ishii, H. Tanobe, Y. Kondo, and Y. Yoshikuni, “A Tunable interdigital electrode (TIE) DBR laser for single-current continuous tuning,” IEEE Photon. Technol. Lett., vol. 7, pp. 1246–1248, 1995. [7] S. Illek, W. Thulke, C. Schanen, H. Lang, and M.-C. Amann, “Over 7 nm (875 GHz) continuous wavelength tuning by tunable twin-guide (TTG) laser diode,” Electron. Lett., vol. 26, pp. 46–47, 1990. [8] Y. Kondo, K. Sato, M. Nakao, M. Fukuda, and K. Oe, “Extremely narrow linewidth ( 1 MHz) and high-power DFB lasers grown by MOVPE,” Electron. Lett., vol. 25, pp. 175–176, 1989.
