High-modulation efficiency operation of GaInAsP/InP membrane distributed feedback laser on Si substrate

The direct modulation characteristics of a membrane distributed feedback (DFB) laser on a silicon substrate were investigated. Enhancement of the optical confinement factor in the membrane structure facilitates the fabrication of a strongly index-coupled (κi = 1500 cm−1) DFB laser with the cavity length of 80 μm and a threshold current of 270 μA. Small-signal modulation measurements yielded a −3dB bandwidth of 9.5 GHz at 1.03mA bias current, with modulation efficiency of 9.9 GHz/mA1/2, which is, to the best of our knowledge, the highest value among those reported for DFB lasers. ©2015 Optical Society of America OCIS codes: (230.0230) Optical devices; (140.3490) Lasers, distributed-feedback; (200.4650) Optical interconnects; (250.5960) Semiconductor lasers. References and links 1. A. 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Introduction
Over the past few decades, advancements in optical communication technology have enabled large-capacity transmission systems from transpacific networks to interconnect with supercomputers in data-centers [1]. Accordingly, optical communications to further shortreach applications are also being investigated. One of the ultimate challenges in this regard is on-chip optical interconnection, which has the potential to overcome the bottlenecks due to delay and Joule heating that occur in the global wiring layer of LSI chips. For such applications, optical devices are required to operate with ultra-low-power consumption [2]. In addition, a small size is needed to integrate many devices on a chip area of a few square centimeters. An effective means of achieving this is to use a strong optical confinement structure. Vertical-cavity surface-emitting lasers (VCSELs) have two high-reflection mirrors that sandwich the active layer. There are many reports on the high-speed and low-power consumption operation of VCSELs [3,4]. However, for use as the light source of an in-plane optical circuit, a mechanism to bend the output beam into the in-plane circuit is needed [5]. Various compact optical devices, such as modulators [6,7] and lasers [8], have been realized by using a microdisk cavity structure. However, the output efficiency of the structure is low because of its evanescently coupled output waveguide. A photonic crystal laser with airbridge structure [9] is an attractive device for an in-plane light source. Although it shows ultra-low-energy consumption operation, avalanche photodiode (APD) was used in this measurement owing to its small output power of several microwatts [10]. In general, APD requires several tens of volts and consumes more energy than a p-i-n photodiode (PD). Therefore for on-chip application, a p-i-n PD, which can operate with a reverse bias of even less than one volt, is a possible candidate. Considering that the minimum receivable power of a p-i-n PD for error free transmission is typically −13 dBm (50 µW) for 10 Gb/s signals, the required output power of the laser becomes on the order of hundreds microwatts. This means that not only low-energy consumption operation but also sufficient output power is essential. In addition to that, efficient modulation characteristics of light source is necessary. For example, to achieve 10 Gb/s operation with a bias current of 1 mA and a bias voltage of 1 V for a pulse energy cost of 100 fJ/bit [2], modulation efficiency (the slope of relaxation oscillation frequency fr against square root of bias current above threshold) higher than 5 GHz/mA 1/2 is required since a pulse modulation speed is approximately limited by 1.3 × 1.55 f r [11]. Moreover an increase of the modulation efficiency leads to higher modulation speed at a certain bias current. Therefore not only a reduction of the threshold current but also an increase of the modulation efficiency is required for low-power consumption operation of light sources for on-chip optical interconnections. For this purpose, an increase of modal gain by adopting a strong optical confinement structure and a reduction of operation current by adopting a small volume of active region are required. We previously proposed and demonstrated a membrane distributed-feedback (DFB) laser as the light source of on-chip optical interconnections [12]. High-refractive index difference was obtained by using a membrane structure comprising a semiconductor core layer sandwiched by dielectric cladding layers. The optical confinement factor of the active region can be enhanced to approximately three times that of conventional double-heterostructure lasers. Furthermore, depending on the grating design, strongly index-coupled DFB lasers with an index-coupling coefficient of thousands per centimeter are attainable. These properties enable us to realize low-threshold and efficient DFB lasers. For current injection into such a dielectric cladding structure, a lateral-current-injection (LCI) structure [13,14] is a reasonable approach. Thus, the benzocyclobutene (BCB) bonding technique was used to form the membrane structure [15][16][17][18][19]. We consequently realized low-threshold current operation of LCI-membrane DFB laser combined with a butt-jointed built-in (BJB) waveguide [20], and its integration with p-i-n-PD [21]. Asymmetric optical output from two output waveguides was obtained by adopting a distributed-reflector (DR) structure with a power ratio between the front and rear side of 6.7 [22].
In this paper, we report on the direct modulation characteristics of membrane DFB laser bonded on a Si substrate. Under a small-signal modulation, a −3dB bandwidth of 9.5 GHz was obtained at a bias current of 1.03 mA, with a record high modulation efficiency of 9.9 GHz/mA 1/2 .

Fabrication
The top view and cross section of the schematic device structure are shown in Figs. 1(a) and  1(b), respectively. A 270-nm-thick InP-based semiconductor layer with SiO 2 cladding layer was bonded on a Si substrate intermediated by a 2-µm-thick BCB adhesive layer. The stripe width Ws of the active region was 0.7 µm. Wet chemical etching was used to form 50-nmdeep gratings with a period of 298 nm on a 100-nm-thick InP cap layer. The DFB section (cavity length, L DFB = 80 µm) was combined with the passive waveguide (length, L WG = 600 µm) in order to integrate a p-i-n PD [21]. The details of initial epitaxial wafer structure and fabrication can be found in [20]. The active layers consisted of strain-compensated GaInAsP 5QWs (five 6-nm-thick 1% compressively strained quantum-wells and 10-nm-thick 0.15% tensile strained barriers) sandwiched by 15-nm-thick optical confinement layers and bottom and top InP cap layers (50-nm and 100-nm-thick each). Selective area epitaxial growth using organometallic vapor phase epitaxy was conducted three times in order to form a passive waveguide, n-InP and p-InP regions. The wafer was bonded on a Si substrate using a BCB adhesive layer, then the InP substrate and etch stop layers were removed via polishing and wet chemical etching, respectively. The surface grating was defined using electron beam lithography. Then, the substrate was cleaved into bar form to measure the light output.

Lasing characteristics
The device was measured under room-temperature continuous-wave (RT-CW) conditions. Figure 2(a) shows light output versus injection current (L-I) characteristics. A low threshold operation with I th = 270 µA was obtained. The threshold current density was 480 A/cm 2 (96 A/cm 2 /well). The external differential quantum efficiency η d was 12%. The maximum output power was 66 µW at an injection current of 950 µA. This is due to the leakage current through butt-jointed coupling section and large waveguide loss of our membrane stripe structure (α WG = 42 cm −1 ) [23]. When the waveguide loss is reduced to around 10 cm −1 , the external differential quantum efficiency from a single side will be improved to be approximately 20%. In addition to that, by introducing the DR structure that consists of both DFB and DBR sections [22,24], the differential quantum efficiency for a single side can be increased to near 40%. Hence we expect that an optical output at the same bias current can be increased to approximately 3 times higher, and it will meet the requirement for light sources for on-chip optical interconnect. Lasing spectra at two different bias currents are shown in Fig. 2(b). The lasing wavelength was 1533 nm at a bias current Ib of 410 µA (1.5I th ). Another lasing mode occurred when the bias current was increased to 660 µA, as depicted by the blue line. In this work as one side of the facet was formed inside the DFB section, the phase of the cleaved facet has been attributed with poor single mode property. The measured stopband width was 32 nm, which corresponds to an index-coupling coefficient of 1500 cm −1 . Thus, the coupling strength of cavity κL was calculated to be 12.

Modulation characteristics
Before performing direct modulation measurements, we measured relative intensity noise (RIN) spectrum of the membrane DFB laser. Since the peak frequency of the RIN spectrum is corresponds to the relaxation oscillation frequency f r , we can estimate the slope efficiency of the relaxation oscillation frequency as a function of square root of the bias current above the threshold (I b -I th ). Figure 3(a) shows RIN spectra for various bias currents measured at room temperature. Figure 3(b) shows the plots of relaxation oscillation frequency f r as a function of the square root of the bias current above the threshold. From linear fitting of measured plots, the slope of the relaxation oscillation frequency of 10 GHz/mA 1/2 was obtained. The small-signal frequency response of membrane DFB laser was measured using a vector network analyzer. The electrical modulation signal was directly applied to the device via a 20-GHz high-speed GS probe without impedance matching. The optical output signal from the device was collected via a spherical-lensed single-mode fiber. The coupling loss between the membrane DFB laser and lensed fiber was approximately −10 dB. The collected optical signal was amplified using an erbium-doped fiber amplifier (EDFA) to obtain optical power receivable by the photoreceiver. Then, the signal was detected by a 20-GHz bandwidth p-i-n photoreceiver. Small-signal frequency responses (S 21 ) for various bias current conditions are shown in Fig. 4. The maximum −3 dB bandwidth of 9.5 GHz was obtained at a bias current of 1.03 mA (3.8I th ). Taking this fact into account, we can expect 10 Gb/s operation with the bias current less than 1 mA. We extracted the damping factor by fitting the results of the small-signal frequency response. The relationship between the damping factor γ and squared relaxation oscillation frequency f r 2 is given by the following equation: where K is the K-factor and γ 0 is the damping offset. Figure 5 shows plots of damping factor versus squared relaxation oscillation frequency. The K-factor and offset γ 0 are 0.24 ns and 5.6 ns −1 , obtained from the slope and intercept of the linear fitting line. The maximum bandwidth is given by From this equation, the available maximum intrinsic bandwidth is calculated to be 37 GHz. As shown in Fig. 2(a), the light output was saturated at the bias current of around 1 mA, and increment of bandwidth also saturated. Namely, the bandwidth of this device was limited by light output saturation caused by thermal heating.  Figure 6 shows the relaxation oscillation frequency f r and −3 dB bandwidth f -3dB as a function of the square root of the bias current. The slope of relaxation oscillation frequency and −3 dB bandwidth are 9.9 GHz/mA 1/2 and 14.8 GHz/mA 1/2 , respectively. The slope of f r well agrees with results of RIN spectrum measurements. Previously reported membrane type DFB laser showed a modulation efficiency of 7.7 GHz/mA 1/2 with a threshold current of 900 µA, and 25 Gb/s direct modulation was demonstrated [25]. Although we didn't demonstrate high-speed large-signal direct modulation, we successfully achieved lower threshold current of 270 µA and obtained the modulation efficiency of 9.9 GHz/mA 1/2 , which is a record high among those of previously reported DFB lasers to the best of our knowledge. Since the active region volume V a of our membrane DFB laser was 1.7 µm 3 (cavity length = 80 µm, stripe width = 0.7 µm, five 6-nm thick quantum wells) which was 0.65 times of that in Ref [25]. (2.6 µm 3 ; cavity length = 73 µm, stripe width = 1 µm, six quantum wells), we think the reason for higher modulation efficiency of our device can be attributed to the smaller volume of the active region because the modulation efficiency is proportional to inverse of square rooted active region volume 1/(V a ) 1/2 . By dividing the modulation efficiency of 7.7 GHz/mA 1/2 in Ref [25]. by (0.65) 1/2 , the modulation efficiency of 9.6 GHz/mA 1/2 is obtained. This value well agrees with our result of 9.9 GHz/mA 1/2 . We expect that the higher modulation efficiency is possible by reducing the active region volume if low threshold current density operation is maintained by strongly index-coupled structure. The modulation efficiency of conventional semiconductor cladding DFB lasers was reported to be 2.3 GHz/mA 1/2 [26] and 4.8 GHz/mA 1/2 [27] for GaInAsP-based MQW DFB laser (cavity length = 150 µm) and AlGaInAs-based MQW DFB laser (cavity length = 100 µm), respectively. Comparing with these values, our membrane DFB laser showed 2-4 times higher modulation efficiency. Since the membrane structure offers about three times higher optical confinement factor of active region than conventional laser structure [12], it leads to high modulation efficiency property of the membrane laser. The roll-off property from the linear plot occurring at (I b -I th ) 1/2 = 0.5 mA 1/2 is due to the second lasing mode, as shown in Fig. 2(b). The decrease in the slope efficiency is caused by gain saturation. We believe that 10 G/s operation can be achieved by this membrane DFB laser with less than 1 mA bias current.

Conclusion
In conclusion, we performed direct modulation measurements of membrane DFB laser bonded on a Si substrate. The device exhibited a low-threshold current operation of 270 µA and an external differential quantum efficiency of 12% from the front facet. Small-signal modulation measurements indicated that a high modulation efficiency of 9.9 GHz/mA 1/2 was obtained. These results demonstrate the benefits of using a membrane structure. Thus, membrane DFB lasers are capable of high-speed and low-power consumption operation as the light source of on-chip optical interconnection applications.