Broadband grating-coupled external cavity laser, based on InAs/GaAs quantum dots, is achieved. The device has a wavelength tuning range from 1141.6 nm to 1251.7 nm under a low continuous-wave injection current density (458 A/cm2). The tunable bandwidth covers consecutively the light emissions from both the ground state and the 1st excited state of quantum dots. The effects of cavity length and antireflection facet coating on device performance are studied. It is shown that antireflection facet coating expands the tuning bandwidth up to ~150 nm, accompanied by an evident increase in threshold current density, which is attributed to the reduced interaction between the light field and the quantum dots in the active region of the device.
©2010 Optical Society of America
Grating-coupled external cavity (EC) laser is an important kind of coherent light source for the applications of spectroscopy , biomedical , interferometry  and so on. Combining with the fast swept frequency technique, this kind of light source can also be applied in the wavelength-division-multiplexing (WDM) system  and optical coherence tomography (OCT) measurement [5,6]. Wavelength tuning range is one of the most important parameters of an EC laser, as a large tuning range increases the channel amount of a WDM system and improves the spatial resolution for the OCT measurement significantly. It was proposed that the characteristic of size inhomogeneity, naturally occurring in self-assembled quantum dots (QDs) grown by Stranski-Krastanow mode, is beneficial to broadening the gain spectra and suitable for the realization of broadband emission . Based on their broadband emission characteristic, QDs have successfully been used for the fabrication of superluminescent diode (SLD) [8–12] and broadband laser diode [13–15]. For 1.5-μm wavelength range, broadband InP based Q-dash laser has also been achieved recently . In the last few years, EC lasers with QDs as gain medium have been demonstrated [17–25]. Compared to the EC laser with quantum well as gain medium [26–29], the QDs’ size inhomogeneity and relatively low ground state (GS) saturated gain make a QD-EC laser being advantageous in low injection current density, broadband and uninterrupted tuning in wavelength. Only utilizing the QD-GS emission, tuning range of 69 nm  and 83 nm  were realized for the QDs devices without facet coating and with single facet antireflection (AR) coating, respectively. By involving QDs’ GS and excited state (ES) transitions simultaneously, a tuning wavelength range of 201 nm  has been reported. The unique characteristics of QD-EC laser make it preferable over other broadband light source, such as QD laser and SLD. Combining with the characteristics of wide wavelength tuning and narrow linewidth, QD-EC laser can be used for the sensitive absorption spectroscopy. Besides, the QD-EC laser can also be used in Fourier-domain OCT system [5,6] based on swept wavelength interferometry, which offers higher sensitivity and imaging speed over conventional time-domain technique.
Besides a tunable bandwidth, continuous-wave (CW) operation of an EC laser at low injection current density is also required in practical applications. Generally, in order to achieve a wide wavelength tuning range, AR coating on the device facet is needed to increase the threshold current density (Jth) of inner Fabry-Pérot (FP) cavity resonance. However, AR coating also increases the EC Jth significantly (the reason will be discussed later) and the CW operation is difficult to be realized [18,19]. Without AR coating, the injection current density can remain at a low level, but the tuning range is restricted [17,23]. Therefore the trade-offs between the two key features of tuning bandwidth and working current of an EC laser are very important for its practical applications.
In this paper, the effect of cavity length on the tuning bandwidth and Jth of a QD EC laser has been investigated. A tuning range of 110 nm (1141.6-1251.7 nm) under 458 A/cm2 CW injection level has been realized for a 2-mm length device without facet coating. The wavelength tuning range covers the QDs’ GS and the 1st ES light emissions simultaneously. The effect of device-facet AR coating on the EC laser’s performance is also studied. As compared with the EC laser employing as-cleaved facet, AR facet coating leads to enhancement of tuning bandwidth and increase of Jth evidently. The latter is attributed to the reduced interaction between the light field and the QD active region.
The epitaxial structure of the QD gain devices used in this study was grown on n-GaAs (001) substrate by a Riber 32P solid-source molecular beam epitaxy machine. Five layers of self-assembled InAs QDs covered by 5-nm In0.15Ga0.85As and separated from each other by 35-nm GaAs spacer form the active region, which is embedded in the waveguide. 2-monolayer InAs is deposited at 500 °C for the formation of QDs in each layer. The areal density is about 4×1010 cm−2 obtained by atomic force microscopy for an uncapped sample, which has the same growth parameters as the device structure. Below and above the waveguide are 1.5-μm n- and p-type Al0.5Ga0.5As cladding layers grown at 620 °C, respectively. Finally, a p +-doped GaAs contact layer completes the structure. The QD epitaxial wafer was processed to fabricate gain devices of broad-area ridge structure with a stripe width of 120 μm and a length of 1-3 mm. They were mounted epitaxial-side down on copper heat sink.
A Littrow configuration is constructed in the EC tuning experiment . The emission from one facet of a QD gain device is nearly collimated by using an aspherical lens, and then is fed back by a 1200-grooves/mm grating in its 1st diffraction order. By rotating the grating, the EC resonance wavelength is selected. The emission from the other facet of the QD gain device is used to perform emission spectra and output power measurements. The device is tested at room temperature without temperature control and with CW (<900 A/cm2) or pulsing (>900 A/cm2, 1 kHz repetition rate and 3% duty cycle) injection. The spectral resolution is about 0.5 nm, lying on the grating monochromator used.
3. Results and discussion
Three QD gain devices, 1 mm, 2 mm, and 3 mm in cavity length, are fabricated. The current-injection emission spectra from the three free-running gain devices are shown in Fig. 1 . As shown in Fig. 1(c), for the device of 3 mm in cavity length under 28 A/cm2 injection, the full width at half maximum (FWHM) of the emission spectrum is 61 nm, which should originate mainly from spontaneous emission of QDs’ GS. This relatively wide GS emission is attributedto the size inhomogeneity naturally occurring in QDs’ growth. Due to the sufficient GS gain, the 3-mm device lases at GS with a Jth of 206 A/cm2. As the significant increased mirror loss with the reduction of the cavity length, 2 mm long device lases at the 1st ES of QDs at 1163 nm. At the injection of 333 A/cm2, simultaneous contribution of QDs’ GS and the 1st ES to emission spectrum leads to a FWHM of 95 nm (Fig. 1(b)). While for the device with 1-mm cavity length, in addition to GS and the 1st ES, the 2nd ES can be filled before lasing, as shown in Fig. 1(a). The inset of Fig. 1 (a) shows a 3-peak Gaussian fitting of the emission spectrum under 833 A/cm2 injection, presenting the GS, the 1st and the 2nd ESs transitions in QDs, respectively. Because the gain of low-energy state transition is too small to compensate for the total loss, lasings occur at the 2nd ES transitions for 1 mm device.
The QD gain devices with different cavity length were put in the Littrow setup to evaluate the tuning properties. The tuning spectra of the devices are shown in Fig. 2 . No facet coating was applied on the device facets. In order to avoid the inner FP resonance, the injection current density is chosen just below the Jth of the free-running gain device. So no inner FP resonance appears even when the wavelength is tuned to the extremes, as presented in Fig. 2. For all the tuning wavelengths, the sidemode suppression ratio is better than 25 dB and the amplified spontaneous emission suppression ratio is better than 20 dB. The FWHM of the lasing spectra is no more than 2 nm. Because the measurements for the spectra and power are performed from one facet of gain device, the spatial distribution of the emitted optical mode should be the same as the free-running device. The gain devices with different cavity length are also different in tuning range. For the device of 3-mm cavity length, there is only contribution from the QDs’ GS and a tuning bandwidth of 55 nm ranging from 1198.2 to 1253.1 nm, is achieved. While for the device with 2-mm cavity length, the wavelength tuning extends to 1141.6 nm at the short-wavelength side, with no sacrifice of long-wavelength tuning. Decreasing the cavity length further down to 1 mm leads to a tuning bandwidth of 100 nm (1073.9-1173.8 nm), contributed by the 1st and the 2nd ESs. The large cavity loss makes the disappearance of the tuning across QD-GS.
Figure 3 compares Jth dependence on the tuning wavelength for the QD gain devices with 1-, 2- and 3-mm cavity length, respectively. The Jth of the three free-running devices without facet coating are 206, 475, and 1708 A/cm2, respectively. The EC laser with 3-mm cavity length shows the lowest Jth (117-194 A/cm2) and the narrowest tuning range. In order to avoid the inner lasing in the device, the injection current density is restricted to a relatively low value and only the GS of QDs can be populated under this injection level. The 1-mm device, which is shortest in cavity length, shows broader tunability at the expense of higher Jth. As shown in Fig. 2(a), the gain device with 1-mm cavity length allows EC laser to work under the injection level up to 1667 A/cm2. Under this injection level, the 2nd ES of QDs in the device can be populated. However, the GS saturated gain cannot compensate the external cavity loss for the 1-mm length device. So the wavelength tuning range covers only the 1st and the 2nd ESs, with the GS foreclosed. The EC laser with 2-mm cavity length shows better performance. Although the Jth increases to some extent, it still remains equivalent tuning to the long- wavelength side compared to that with 3-mm cavity length device. Furthermore, the tuning on the short wavelength side can be extended to the 1st ES.
It can be seen from the above results that the choice of the cavity length is crucial for optimization on both Jth and the tuning bandwidth. On the premise that the GS saturated gain is higher than the external cavity loss, the cavity length of the gain device should be as short as possible. Thus, the tuning range can cover both the GS and the 1st ES and the tuning range above 100 nm can be realized. Besides, the low QDs’ GS saturated gain makes the QDs’ ESs be occupied under lower injection level. This gives rise to lower Jth at the short wavelength side in the tuning range compared to the QW-EC laser. Generally more than 10 kA/cm2 pump level is needed for the carriers filling to the ESs in QW-EC laser [26,27]. In addition, the spectral broadening of QDs induced by its size inhomogeneity is beneficial to the consecutive tuning between two neighboring states.
Figure 4 shows the output power as a function of tuning wavelength for the QD gain devices of 1-, 2- and 3-mm in cavity length. It can be seen from the figure that under the given injection level as used in Fig. 2, the maximal output power is 65, 53 and 54 mW and the corresponding working current is 2-, 1.1- and 0.7-A, respectively. Although the output power is comparable for the three devices, the working current shows great difference and the 3-mm device has the lowest power consumption. This indicates that the longer device possesses higher efficiency. From the figure, it also shows that the dependence of the output power on the wavelength represents approximately the gain characteristic of the three QD devices. A single peak can be observed in the output power vs wavelength for the 3-mm cavity length device, which is attributed to the QDs’ GS. However, there are two peaks in the output-power curve for both the 1- and 2- mm cavity length devices, because of the simultaneous contribution of two states. For the 2-mm cavity length device, the output power at GS is lower than that of the 1st ES due to finite gain of GS. Increasing the cavity length of the device slightly can eliminate the difference in output power for the two QDs’ states to some extent.
AR facet coating of a gain device has been proved to be effective to enlarge the tuning bandwidth [18,19]. Because the Jth of the inner FP resonance can be increased effectively by facet coating, the EC laser can work under a relatively high injection level and the tuning towards short wavelength can be realized easily. In order to evaluate the effect of AR coating, a single λ/4 ZrO2 layer designed for minimum reflectivity at 1227 nm was deposited on one facet (coupled with the grating in the tuning experiment) of the gain device 3 mm in cavity length. After coating, the device lases at 1133 nm due to the increased mirror loss. From the difference in slope efficiencies between the two facets, an effective reflectivity of approximately 2.7% is estimated at the lasing wavelength.
A ~150 nm tuning bandwidth (1084.7 nm-1234.3 nm), covering simultaneously the GS, the 1st and the 2nd ESs, is shown for EC tuning with the 3-mm facet coating device . The Jth dependence on the tuning wavelength is also presented in Fig. 3. Because of the increased facet loss, the Jth of the EC laser at 1228 nm with AR facet coating (319.4 A/cm2) is 2.6 times as high as that without coating (122.2 A/cm2). Accordingly, the CW mode operation cannot be ensured across the whole tuning range. The increase of Jth can be explained as follows. With AR coating on the device facet, more radiation comes out from the active region and enters into the EC, where no active medium exists. Light field in the EC doesn’t interact with the injected carriers and shows no contribution to the stimulated radiation. Namely, the optical confinement factor in the direction of light transmission decreases by the facet AR coating. As a result, the Jth increases. By further optimizing the structure of active region and choosing appropriate cavity length of gain device, an even lower working current density and wider tuning bandwidth should be achieved without facet coating.
In conclusion, the tuning characteristics of QD gain devices with different cavity length is investigated. A 110 nm tuning bandwidth under the low CW injection level of 458 A/cm2, has been realized by using a device of 2 mm cavity length without facet coating. The easiness of GS optical gain saturation is beneficial to the short wavelength tuning with low injection level. The AR coating is also used to expand the tuning bandwidth. A 150 nm tuning band can be realized after AR coating, accompanied with an evident increase of Jth, which is attributed to reduced interaction between the light field and the QD active region.
The authors would like to thank Dr. J. Wu for the correction on grammar and P. Liang, H. Sun and Y. Hu for assistance on device fabrication. This work was supported by the National Basic Research Program of China (No. 2006CB604904) and the National Natural Science Foundation of China (Nos. 60976057, 60876086, 60776037 and 60676029).
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