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Abstract
Dual-wavelength lasers with separation from several nanometers to tens of nanometers at 1 µm waveband are attractive light sources for terahertz generation. This work reports a compact dual-wavelength vertical-external-cavity surface-emitting laser with simple elements. The gain chip is regularly designed and epitaxially grown, and the laser cavity is the most common straight line cavity. By the use of a blade as the tuning element in cavity, the laser wavelength can be tuned continuously, and the stable dual-wavelength oscillation can be obtained when the blade is situated at a certain position. The total output power of 85 mW, which is limited by our pump source, is produced when the laser wavelengths are at 961 nm and 970 nm. We have also analyzed the evolution mechanism and the stability of this dual-wavelength laser.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Dual-wavelength lasers are of considerable demand in applications such as frequency down-conversion [1], two-wavelength interferometry [2], free-space wavelength-multiplexed optical communication [3], optical distribution and generation of high-purity microwave signals [4] and so on. Besides common dual-wavelength solid state lasers [5], semiconductor lasers are also suitable for dual-wavelength running.
Theoretically, two questions should be solved well prior to obtaining dual-wavelength oscillation in a semiconductor laser: the interaction of optical fields of the different two wavelengths in active region, and the competition of possible modes in cavity [6]. The former mainly involves the absorption of shorter-wavelength laser by longer-wavelength gain medium, which can be avoided by physically isolating longer-wavelength gain medium from shorter-wavelength gain medium [7], or can be solved by using broadband gain medium combined with selective feedback generated by grating and other elements [8]. Coupled cavity can provide a relatively simple and compact geometry of dual-wavelength semiconductor laser [9], but in order to weaken the interaction between optical modes, the coupling effect of cavity must be weak, which means that the interval between two wavelengths is very limited. Except for the method of coupled cavity, those means used to obtain dual-wavelength output from semiconductor lasers will all complicate the structure and thus enlarge the size of laser. Although a dual-wavelength semiconductor laser with coupled cavity can be compact and miniaturized, the mode competition would make laser unstable, and the wavelength separation available is generally less than 30 nm.
As a new kind of semiconductor lasers, vertical-external-cavity surface-emitting lasers (VECSELs) combine advantages of both semiconductor surface-emitting lasers and solid state disk lasers, can produce high power and good beam quality simultaneously [10–14]. The intrinsic design flexibility of VECSELs; i.e., the wideband tailorable gain spectrum and the convenient changeable external cavity, can be utilized for realizing operational requirements of dual-wavelength laser sources.
Different ways have been introduced in a VECSEL to force the laser operating at two wavelengths before. The early reported dual-wavelength VECSEL used two kinds of quantum wells (QWs) with different composition in a single gain chip, and 58 nm wavelength separation was achieved [15]. After that, a two-wavelength VECSEL with 2.1 nm wavelength interval was presented utilizing a tilted intracavity Fabry-Perot etalon and a Brewster window [16]. Since a single gain chip cannot provide wide tunability under dual-wavelength operation, two different gain elements are necessary for the purpose of wavelength tuning. Dual-color output with tunable wavelength separation from 35 to 52 nm was demonstrated in a two-chip collinear T-cavity VECSEL [17], while another dual-wavelength emission provided 10 nm wavelength separation and 17 nm tuning range from a serially-connected two-chip VECSEL [18]. In addition, dual-wavelength VECSELs also have been realized by using a multiple folded cavity [19], by employing a spatially separated mode in the gain medium [20], or by utilizing a diffractive grating and a dual-feedback-configuration cavity [21].
We have demonstrated a dual-wavelength VECSEL for wideband tunable mid-infrared difference frequency generation before [22]. In this work, a two-color VECSEL with wavelength interval of more than 10 nm at 1 µm waveband, which is of great interest in THz wave generation [23–25], is focused. In contrast to previous dual-wavelength designs, we propose a convenient intra-cavity physical aperture; i.e., a blade, as the mode discriminator to balance the competition between two longitudinal modes and so to support dual-frequency operation. The laser is monolithic, rather simple, compact, and robust and with an exactly coaxial output beam, that are all highly desired in applications.
2. Gain chip
The gain chip used in the experiment is epitaxially grown in reverse sequences. Firstly, an etch stop layer of AlGaAs with high Al composition is deposited on GaAs substrate, then a protect layer of GaAs is grown. An AlGaAs layer with high barrier to prevent the carriers from surface recombination comes next, and the following is the active region consisting of multiple quantum wells. There are twelve InGaAs/GaAsP quantum wells in the active region, and the content of In in InGaAs is designed to meet the target laser wavelength of 980 nm. Since the GaAsP layer would play three roles (i.e., the strain compensation layer, the barrier layer, and the pump absorbing layer) in the active region, so the content of P in GaAsP must be adequate to compensate the strain, and meanwhile, cannot be too much to absorb the pumping energy.
Above the active region is the distributed Bragg reflector (DBR), which is composed of 30 pairs alternate AlGaAs layers with high Al (lower refractive index) and low Al (higher refractive index) composition. The designed center wavelength and high-reflectivity bandwidth of the DBR are 980 nm and 100 nm respectively. The entire epitaxial wafer is ended by an antioxidant GaAs layer. In a VECSEL, the DBR at bottom and the semiconductor-air interface at front of the chip form a microcavity, and this will result in a laser standing wave in the active region. In order to get higher gain coefficient, the thickness of every single layer in the wafer, especially layers of multiple quantum wells, should be designed and grown accurately to ensure that all quantum wells are located at the peaks of the standing wave, so to form a resonant periodic gain structure [26].
The grown wafer is split to small chips with 4 mm × 4 mm dimension. The epitaxial end face of chip is metalized with titanium-platinum-aurum sequentially, then the chip is bonded to a copper heatsink, and the substrate is removed using chemical etch.
Figure 1 shows the schematics of the experiment. A high-reflectivity coated (for 980 nm wavelength) plane-concave mirror is used as the output coupler (OC), and the so-called external-cavity; i.e., the laser resonant cavity, is formed by the DBR at bottom of gain chip and the OC. The final output modes of laser are the overlapping part of the external-cavity modes and the microcavity modes described before, and these modes also should meet the threshold conditions of the laser at the same time.
To get characteristics of the gain chip, we collect the edge- and surface-emitting photoluminescence (PL) spectrum in the parallel and vertical direction of the chip respectively, and the measured spectra under 1.6 W pump power and 12°C temperature are drawn in Fig. 2. In order to examine whether the epitaxial qualities are met our design targets, the theoretically calculated material gain of quantum wells in active region of gain hip is also plotted in Fig. 2.
Fig. 2. Edge- and surface-emitting PL spectrum of the gain chip under 1.6 W pump power and 12 °C temperature. The calculated material gain of multiple quantum wells is also plotted.
Procedure for gain simulation is as following. Firstly, the famous k·p method [27–29] is used to calculate the band structures: the conduction band structures are treated using a parabolic approximation and the confined energy levels are obtained by solving the Schrödinger equation, and the valence band structures are computed using six-band Luttinger-Kohn Hamiltonian [30]. Then the Lorentzian line-shape function based material gain of quantum wells is derived from Fermi’s golden rule:
In simulation, the increasing temperature caused bandgap reduction and the carrier density related bandgap renormalization are taken into account. Details for calculation and parameters used in the modeling can be found in [31].
It can be seen from Fig. 2 that peak wavelengths of the edge-emitting PL spectrum, the surface-emitting PL spectrum and the material gain overlap well near 961 nm, which means that the epitaxial growth are in good agreement with our design targets. It should be noted that because the emitting wavelength of InGaAs quantum wells will redshift with a rate of about 0.3 nm/°C when the VECSEL works under intense pump hence the temperature in the active region goes up, the designed emitting wavelength of InGaAs quantum wells under weak pump (or under room temperature) is 965 nm instead of 980 nm, so when the quantum wells are heated to the expected value of 75°C by the rising pump, its emitting wavelength shift just to the desired laser wavelength of 980 nm. Figure 2 indicates that the emitting wavelength of quantum wells under weak pump is 961 nm, close to the designed value of 965 nm.
Edge-emitting PL spectrum reflects the intrinsic luminescence properties of quantum wells. As shown in Fig. 2, the envelope of edge-emitting PL spectrum is consistent with the gain curve. However, there are three obvious narrow peaks in the spectrum, and the highest peak near 961 nm has a full-width-half-maximum (FWHM) of 3 nm, which means a strong amplified spontaneous emission (ASE, or named lateral lasing) in the gain chip [32]. To a certain extent, this in-plane ASE in the active region will deplete carriers in conduction band thus limit the output power of the laser.
Compare to edge-emitting PL spectrum, the surface-emitting PL spectrum will carry information of microcavity, and the three cavity modes at 925 nm, 961 nm and 997 nm in the spectrum are clearly shown in Fig. 2. In view of that the oscillating wavelength in a VECSEL essentially depends on the surface-emitting spectrum, so we investigate the changes of surface-emitting PL spectra under different pump power and various temperatures, and the measured results are presented in Figs. 3–6.
Fig. 3. Surface-emitting PL spectra of the gain chip under 1.6 W pump power and different temperatures.
Fig. 4. Relationship between the peak wavelengths of surface-emitting PL spectrum and temperatures of the gain chip under 1.6 W pump power.
Fig. 6. Peak wavelengths of the surface-emitting PL spectrum as function of pump powers on the condition of 12°C temperature.
Figure 3 shows the surface-emitting PL spectra of gain chip under 1.6 W pump power and different temperatures, and relationship between the wavelengths of the highest peak in surface-emitting PL spectra and the different temperatures of gain chip is depicted in Fig. 4. The 0.1 nm/°C shift rate of wavelength is in line with the change rate of the microcavity length, which is primarily caused by the change of refractive index of GaAs based materials with varying temperature.
Surface-emitting PL spectra of gain chip with 12°C temperature and various pump powers are shown in Fig. 5, and the corresponding wavelengths of the highest peak in surface-emitting PL spectra as function of pump powers can be seen in Fig. 6. Considering Figs. 4 and 6 together, the 0.4 nm/W slope in Fig. 6 suggests that the temperature of the active region rises by 4°C for every 1 W increase in pump power. This indicates a relatively serious thermal effect in the laser, which would partly limit the output power of our VECSEL, and the reason of why is: the temperature of active region increases because of the thermal effect, and the gain of quantum wells decreases sharply with the increasing temperature. Meanwhile, the laser wavelength redshifts so the periodic resonant gain structure in the active region will be detuned. In addition, the nonradiative recombination of the carriers will become dominant and the temperature rise will be further accelerated. All of the above effects are superposed upon the laser until finally the thermal rollover of the laser occurs.
3. Dual-wavelength operation
For some reasons, such as the electron-phonon interaction, the width and composition fluctuations in multiple quantum wells, or the material defects [16], the gain spectrum of a VECSEL is inhomogeneously broadened to generally more than 20 nm bandwidth with relatively flat top. Therefore, it is quite possible for a non-specially designed VECSEL to simultaneously oscillate at two wavelengths separated by a few nanometers to tens of nanometers, if we can introduce an adjustable mechanism to decrease the gain (or increase the loss) of an existing mode, so to reduce its competitive advantage, thereby another mode that has been suppressed previously would have the opportunity to oscillate and coexist with it in the cavity.
Here we use a blade, which can be considered as a half slit aperture, as the above mentioned loss-adjustable element. The regulation mechanism for the loss of modes in a VECSEL is straightforward: in a simple linear cavity shown in Fig. 1, the mode size; i.e., the diameter of laser spot, can be expressed as
where λ is the laser wavelength, L is the length of the observation point from the gain chip, and R is the curvature radius of output coupler. It is very clear that the mode with longer wavelength has a lager mode size than the mode with shorter wavelength. So, when a blade is inserted perpendicularly to the optical axis in the cavity, it introduces loss to the mode with longer wavelength more than to the mode with shorter wavelength, and this may inhibit an existing longer-wavelength mode and stimulate a previously suppressed shorter-wavelength mode to oscillate, or, allow the tow modes lasing simultaneously in the cavity if both modes have the same mode gain.Figure 7 shows the evolution of the laser wavelengths from longer one-wavelength to dual-wavelength and finally to shorter one-wavelength. We plot laser wavelength on the horizontal x-axis against the spectral intensity on the vertical z-axis. The horizontal y-axis represents the position (i.e., the insertion depth) of the blade. Since the oscillation wavelength of laser will change continuously to shorter wavelength when the insertion depth of blade is increased gradually, for convenience, we choose the corresponding position of blade as the coordinate origin of y-axis when the laser wavelength just begins to change.
Fig. 7. Evolution of the laser wavelengths from longer one-wavelength to dual-wavelength and finally to shorter one-wavelength.
It can be seen form Fig. 7 that when the blade is inserted into the cavity, before the simultaneous two-wavelengths is established, that is, when there is only one oscillating mode in the cavity, the laser wavelength can be continuously tuned with the increase of the insertion depth of blade, as shown in the first six wavelengths. After the dual-wavelength oscillation occurs, the insertion depth of the blade mainly changes the relative intensities of the two wavelengths, but not the two wavelengths themselves and the space between the two wavelengths.
The two distinguishable modes appear in cavity when the insertion depth of blade exceeds 80 µm. After that, the longer-wavelength mode is weakened gradually while the shorter-wavelength mode is enhanced simultaneously with increasing insertion depth of the blade. The stable balanced dual-wavelength oscillation appears at the position where the insertion depth of blade is 120 µm. Beyond this position, the longer-wavelength mode is attenuated and the shorter-wavelength mode is amplified continuously until the position of blade reaches 140 µm, in which only a single shorter-wavelength mode is left to oscillate in the cavity.
In the experiment, the radius of curvature of the output coupler is 100 mm. Because the diameter of pump spot on the gain chip is about 200 µm, for the sake of mode-matching, the length of laser cavity is chosen to be 90 mm, so the size of laser spot on the gain chip matches the size of pump spot well. In above conditions, the diameter of laser spot on the output mirror is about 580 µm. To get better mode discrimination ability, we put the blade as close as possible to the output mirror.
As shown in Fig. 7, the stable balanced dual-wavelength oscillation is achieved when the insertion depth of the blade is approximately 120 µm. For a Gaussian beam whose intensity satisfies the following equation
where I0 is a constant, r is the radial coordinate, and ω is the radius of the beam waist. The transmittance of a Gaussian beam through an aperture with radius of a can be expressed asIt can be estimated that when the laser is in stable balanced dual-wavelength operation, the transmittance of laser beam is reduced by about 36% due to the insertion of the blade compared with the initial single-wavelength operation. The two oscillating wavelength in Fig. 7 are 961 nm and 970 nm, corresponding to a frequency difference of 2.9 THz.
It should be noticed that the wavelength of VECSEL can certainly be tuned to a shorter wavelength continuously with the increasing insertion depth of blade. However, the stable balanced dual-wavelength output cannot be obtained always. As shown in Fig. 8, when the initial oscillating wavelength (this wavelength is determined by several factors including the surface-emitting PL spectrum of gain chip, the reflection spectrum of DBR, and the coating of output mirror) of VECSEL is on the longer-wavelength side of the surface-emitting PL spectrum, there may be a shorter-wavelength mode with the same gain as the existing longer-wavelength mode during the inserted blade gradually weakens the gain of the latter to a certain extent. Only in this way, can the two wavelengths coexist and oscillate in the cavity simultaneously.
Fig. 8. Laser spectrum of the two-wavelength VECSEL and the surface-emitting PL spectrum of gain chip.
Since there is no specific beam splitter to separate the two wavelengths, we measured the total output power of the dual-wavelength VECSEL under condition of 16°C temperature, and the results are shown in Fig. 9. The pumping threshold of the laser is 2.9 W, and due to the relatively large intracavity loss caused by the inserted blade, the slope efficiency of 2.1% is a little bit small. The optical-to-optical efficiency of the laser is about 1.5%. Figure 9 shows the maximum output power of the laser is 85 mW, which is limited by the supply of our pump source. It can be expected that the output power of this kind of dual-wavelength VECSEL can be improved to a practical level by higher pump.
Fig. 9. Total output powers of the dual-wavelength VECSEL vs pump powers under condition of 16°C temperature.
Because the two laser modes cannot be separated as mentioned before, we measure the spectral intensity instead of the output power of the two wavelengths individually to characterize the stability of the dual-wavelength VECSEL. In order to make the drawing more convenient, we intentionally adjust the insertion depth of the blade to unbalance the intensity of the two wavelengths. The measured results are shown in Fig. 10. It can be seen from Fig. 10 that the stability of the longer-wavelength mode is less than 3% while the stability of the shorter-wavelength mode is less than 3.8% over 300 minutes, and both modes show good stability.
Fig. 10. Stability of the dual-wavelength VECSEL over 300 minutes characterized by the corresponding intensities of the two laser spectra.
The instability of dual-wavelength output is mainly caused by the fluctuation of temperature in the active region. It is well known that the optical properties of semiconductors are very sensitive to temperature. In the experiment, we did not use special device to control the temperature. The gain chip is bonded with the heat sink, which is cooled by circulating water. Embarrassingly, the temperature control ability of the water cooler is poor, which is about ±3 °C. The stability of the two wavelengths shown in Fig. 10 can be considered at the same level, but the stability of the shorter-wavelength mode is slightly worse. This is understandable, because the shorter-wavelength mode is in the position where the surface emission spectrum is stronger (can be seen from Fig. 8), its corresponding radiative transition will be affected more by the temperature change, and so does the spectral intensity.
Finally, the influence of inserted blade on the linewidth of laser spectrum is worth to be mentioned. Figure 11 shows the laser spectral linewidths without and with a blade in the cavity, which are 1.6 nm and 1.1 nm, respectively. Obviously, the employment of the blade narrows the linewidth of laser spectrum slightly, and this can be explained that the inserted blade limits the transverse mode of the laser to a certain extent, thus improves the longitudinal mode of the laser partly. Because the inserted blade will reduce the transverse mode of the laser, as well as the laser spot on the gain chip. This will improve the uniformity of the lasing region in the gain chip, reduce the line broadening of the laser, and produce a narrower output laser spectrum.
4. Conclusions
In summary, we have demonstrated a compact dual-wavelength VECSEL by using a general gain chip, a simple straight line cavity, and a handy blade as the tuning element. When the initial wavelength of the laser is located at the longer-wavelength side of the surface-emitting PL spectrum, the gain of the existing longer-wavelength mode would be partly restrained by an inserted blade. As long as this restraint reaches a certain extent, the gain of a previously suppressed shorter-wavelength mode would be equivalent to the gain of the existing longer-wavelength mode. Therefore, both modes may coexist and oscillate in the cavity stably. Due to the limitation of pump power, the maximum output power of the dual-wavelength VECSEL is 85 mW, and the frequency difference of the two wavelengths corresponds to 2.9 THz. The relative intensity of the two modes in the dual-wavelength VECSEL can be adjusted easily by changing the insertion depth of the blade, and both oscillating modes show good stability. This kind of dual-wavelength laser can avoid complicated wafer design and sophisticated epitaxial growth, meanwhile, a hard-to-adjust cavity with many optical elements is also no need for this laser. Its compactness, robustness, stability and practicality would make it attractive in some applications.
Funding
Chongqing Municipal Education Commission (KJQN201800528, KJZD-M201900502); National Natural Science Foundation of China (61904024); Chongqing Science and Technology Commission (cstc2018jcyjAX0319); Ministry of Education of the People's Republic of China (CXZJHZ201728); State Key Laboratory of Luminescence and Applications (SKLA-2019-04).
Disclosures
The authors declare no conflicts of interest.
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