Electrically pumped efficient broadband CW frequency conversion in diode lasers using bulk $χ2$

Optical nonlinear frequency conversion in semiconductors has enabled significant advances in ultrafast signal processing¹ and microwave photonic functionalities.² These play crucial roles in sensing, broadband spectroscopy, and high-performance communication applications.³ Currently, the primary platforms for harnessing optical nonlinearities for the aforementioned applications are based on silicon and Periodically Poled Lithium Niobate (PPLN). In those systems, optical parametric amplification (OPA)⁴ and the optical parametric oscillator (OPO) on a chip^5,6 have been deployed in numerous applications.

Devices such as OPAs and OPOs in these platforms cannot be seamlessly incorporated into a monolithic integrated platform, as they require an external pump source for the nonlinear process. For the purpose of fully integrated photonic circuits, a hybrid integration platform is the main route that provides an integration pump source for PPLN or silicon by leveraging the well-developed laser technology in the compound semiconductor. However, this approach severely limits the scalability of the system. In contrast, the compound semiconductor, such as GaAs, has a strong second order nonlinearity χ² readily available and is capable of pump light generation. However, it is underutilized due to challenges in phase matching. Therefore, an efficient and robust route for harnessing this large χ² nonlinearity available in monolithic semiconductor can offer novel and strategic possibilities for efficient and compact all-optical signal processing circuits.

Gallium arsenide (GaAs), a III-V semiconductor, possesses a strong χ² and has monolithic pump laser capabilities to enable self-pumped nonlinear optical circuits for telecommunication. However, there is extensive dispersion near the material bandgap, where the χ² nonlinearity is strongest. This strong dispersion, in combination with the lack of intrinsic birefringence, poses a challenge in achieving phase matching for efficient nonlinear interaction. To mitigate the dispersion near the bandgap, intraband χ² has been utilized and demonstrated nonlinear conversion.⁷ Utilization of resonant intraband χ² in the mid-IR has resulted in self-pumped frequency conversion, where a quantum cascade laser (QCL) is used as both a pump light generation platform and a nonlinear medium for intracavity frequency generation.^8,9

On the contrary, for utilizing χ² nonlinearities near the bandgap, a few approaches have been developed to achieve phase matching in such a highly dispersive medium, including form birefringence,¹⁰ quasi-phase matching,¹¹ and modal phase matching. Among these approaches, form birefringence is the most developed for nonlinear conversion; however, it lacks the ability to perform electrically pumped intracavity frequency mixing.

Recent successes in utilizing the bulk-coefficient χ² within Bragg laser diodes have enabled optical parametric generation (OPG) through modal phase matching.¹² In this letter, we utilize our recent achievements in this field and demonstrate electrically pumped continuous wave (CW) intracavity difference frequency generation (DFG) in AlGaAs diode lasers with higher efficiency and bandwidth. As illustrated in Fig. 1(a), the optical nonlinear process takes place within the laser diode cavity. The nonlinear parametric process is pumped by optical generation through current injection of the diode laser. With the exact modal phase matching provided by the Bragg reflection waveguide design, difference frequency generation (DFG) is facilitated by coupling a signal into the device and an idler is generated within the same diode laser cavity.

The device discussed in this work is based on AlGaAs heterostructures, as shown in Fig. 1(b). The structure is grown with metal-organic chemical vapor deposition (MOCVD) on an n-type GaAs substrate. A 1D Bragg stack design is utilized to confine light vertically and facilitate a dispersion engineered design for phase matching.¹³ It consists of 5 bottom (3 × 10¹⁸ cm⁻³, n-doped) and 4 top (3 × 10¹⁸ cm⁻³, p-doped) Al_0.7Ga_0.3As/Al_0.3Ga_0.7 bilayers. The active region is designed with two InAlGaAs quantum wells (QWs), which are separated by Al_0.3Ga_0.7As barriers. The wafer described is then used to fabricate Fabry Perot (FP) edge emitting ridge lasers. A cross-sectional scanning electron micrograph (SEM) of one such laser is shown in Fig. 1(c). This device is designed to achieve lasing in the Bragg mode^14,15 while simultaneously providing efficient type-II DFG interaction. Based on numerical analysis for the designed structure, the device has an effective χ² nonlinearity, d_eff, of 36 pm/V. At the phasematched condition, the theoretical normalized nonlinear conversion efficiency is calculated to be 83% W⁻¹ cm⁻². The theoretical tuning range for the signal and idler generation can be approximated by the phasematching condition at the degenerate position, ω_o = ω_p/2, with the following equation:

where β′_TE(TM) is the inverse of group velocity, $vg−1$⁠, for TE(TM) polarization and β″_TE(TM) is the group velocity dispersion (GVD) parameter for TE(TM) polarization. By numerically extracting the dispersion parameters of the corresponding modes, the type-II tuning curves with a fixed pump wavelength analyzed for both polarization inputs are shown in Fig. 2(a), spanning approximately the 140 nm tuning range at this operation point, showing a preferential usage of TM-polarized for the shorter wavelength and TE-polarized for the longer wavelength. (See the supplementary material for more details in the numerical analysis.)

A representative Luminescence-Current-Voltage (LIV) relationship of this laser in the continuous operation mode is shown in Fig. 2(c), showing a similar lasing characteristic as a normal FP diode laser while incorporating a 1D photonic crystal structure. The experimental setup for type-II frequency conversion is shown in Fig. 2(b). With an injection current of 78 mA and stage temperature of 48 °C, the diode laser is phasematched while remaining in a single mode. By injecting a TM-polarized signal into the device, a conversion efficiency between the signal and the idler of −44 dB has been observed with an output pump power of only 1.1 mW in a 1 mm length device. With the signal loss measured to be 10.5 cm⁻¹ and the average intracavity power extracted to be 0.82 mW, the internal normalized DFG conversion efficiency, defined as η = P_idler/(P_pumpP_signalL²),¹³ is calculated to be 169% W⁻¹ cm⁻² or 17 mW W⁻² when not normalized to the length.

To investigate the tunability of the self-pumped DFG process, a range of signal wavelengths are injected into the device and the idler is monitored with the optical spectrum analyzer at a fixed injection current and stage temperature. At a fixed operating condition and a fixed pump wavelength (790.5 nm), DFG is demonstrated for the signal from 1486 nm to 1580 nm, and idler generation from 1686 nm to 1582 nm, which corresponds to a large frequency tuning of 12 THz or 50 meV, as can be seen in Fig. 3(a). Due to the limitation of the wavelength range provided by our signal source, we demonstrated a tunable bandwidth of approximately 100 nm (200 nm total if the idler is used as the input). The full conversion bandwidth can be exploited if a more widely tunable laser is used as the signal to generate a much wider range of idler wavelengths.

A different device is utilized for signal power tuning analysis as well as the injection current tuning analysis. When tuning the signal power with the fixed injection current and device temperature, the generated idler experiences a linear relationship with the injected signal power, as shown in Fig. 3(b). The result for tuning the injection current of the Fabry Perot laser is shown in Fig. 3(c). Changing the injection current initiates three different changes in the DFG operation. Increasing the injection current of the device will result in an increase in the intracavity pump power. However, it will also result in lasing wavelength red-shifting due to a rise in junction temperature. The phasematching condition also red-shifts due to the temperature-refractive dependence. As the combined result of all three behaviors, there is a shift toward the longer wavelength for the idler as the current increases. A similar level of conversion efficiency is maintained due to the increase in intracavity power as well as the shifting of the phasematching condition. Also at 60 mA, three distinct idlers are generated as the Fabry Perot laser enters a multimode lasing regime, limiting the conversion efficiency.

In this work, we presented the first diode laser, which serves as a pump source for the intracavity CW DFG process for the telecommunication band wavelength. Using the bulk χ² nonlinearity inherent to the GaAs material and Bragg laser design, electrically pumped intracavity DFG for 1550 nm was demonstrated. We measured the normalized internal conversion efficiency of 169% W⁻¹ cm⁻² by taking into account the propagation loss. The frequency conversion in this device is suitable for any modulation format as the nonlinear conversion is an ultrafast process.

To the best of our knowledge, this is the first demonstration of monolithic, guided-wave self-pumped DFG light generation in semiconductor lasers with such bandwidth at this wavelength region. Such broad bandwidth is afforded due to the low-birefringence and low group-velocity mismatch of the multilayered AlGaAs material system. As a result, the cross-polarized signal and idler experience very similar dispersion and their corresponding refractive indices do not differ from each other significantly. Theoretically, the normalized conversion efficiency based on the design structure is calculated to be 83% W⁻¹ cm⁻² with the theoretical DFG bandwidth of 140 nm. This discrepancy in nonlinearity can be a result of the combined effects from the resonance within the cavity as well as the local temperature effect that are currently unaccounted for. The discrepancy in the DFG bandwidth can be originated from the complex dynamics of the injection carriers along with fabrication uncertainty. These introduce a deviation of the dispersion from the refractive index model that is being used in the theoretical calculation.

Previously, an electrically pumped QCL-DFG conversion efficiency of 0.6 mW W⁻² was reported for a 1.15 mm device.⁹ When normalized to the length, the normalized internal conversion efficiency of the QCL-DFG is estimated to be 4.5% W⁻¹ cm⁻². The efficiency reported in this work is more than 35 times stronger than that of the QCL-DFG, which utilizes resonant intraband⁹ χ² of 4 × 10⁵ pm/V.

There is also more room for improvement of the conversion efficiency reported here, where we can achieve higher conversion efficiency by utilizing high-reflection facet coating to promote a highly resonant pump cavity for the conversion. The reason for the higher performance in our self-pumped device, in comparison with QCL-DFG with much higher nonlinearity (about 10³ times),⁹ can be attributed to two primary reasons: the modal overlap and the phase-matching scheme. The high nonlinearity for the intraband originates from the resonant quantum wells in the QCL work. As a result, the device efficiency strongly depends on the overlap between the quantum well structure and the modal profiles of the participating waves. Given the large modal area of the waves participating in QCL-DFG, the effective nonlinearity will be lowered. In addition, the terahertz generation in the QCLs employs a Cherenkov phase-matching scheme, where the idler wave is generated at a Cherenkov angle into the bottom cladding. This reduces the overall conversion efficiency, as the idler experiences a high leakage loss. This hinders the possibility of making such a design a guided-wave structure in a monolithic setting, in contrast to the fully guided approach with potential of the highly resonant cavity for both the pump and the signal.

The maximal attainable idler power in this work is limited by the available power in the fundamental pump mode. In our current generation of the device, higher current injection can easily result in multimode lasing (see the supplementary material). This broader output spectrum will then limit the conversion efficiency as there is a finite pump phasematching bandwidth of approximately 1 nm. Multiple longitudinal modes will also generate multiple idlers, which is usually undesirable. A single-mode device, such as distributed feedback (DFB) or distributed Bragg reflector (DBR) grating designs, would be capable of alleviating such a limitation, providing a pump with high spectral density and high side mode suppression. In addition, as only the intracavity power is of concern with the nonlinear conversion, a high Q cavity can greatly enhance the pump photon density for the nonlinear conversion. These design approaches in the future can strongly increase the nonlinear conversion efficiency and pave a strong candidate of developing into an electrically injected OPO through mode-locking and high Q resonators.^14,16 Nonetheless, the current device demonstrates a critical milestone to effectively utilize the bulk χ² process for telecommunication wavelength by electrically exciting the pump mode. This development will allow for a new generation of monolithically integrated all optical signal processing devices for applications in both quantum and classical domains. The nonlinear performance using this architecture, having been demonstrated using merely a 1.1 mW continuous wave pump, attests to the high efficiency of these nonlinear devices.

To conclude, we have demonstrated a widely tunable, continuous-wave, electrically pumped intracavity DFG process in a dc-biased monolithic AlGaAs semiconductor diode laser for conventional telecommunication wavelength. The demonstrated device has −44 dB conversion efficiency for DFG with approximately 1.1 mW pump power. A 200 nm-wide frequency conversion window centered at 1580 nm was demonstrated for the DFG process, confirming a widely tunable device with broadband phase-matching ability. The normalized conversion efficiency is 169% W⁻¹ cm⁻². Further development and progress in laser diode design, such as the implementation of DFB, DBR, and other high Q cavities, would allow even higher nonlinear performance as this will increase the photon density within the cavity. This design with the integrated pump for the nonlinear process is not merely limited to frequency conversion in telecommunication. Its nonlinearity and integrated pump can empower a wide range of applications such as phase regeneration in all-optical processing³ and phase-sensitive amplification¹⁷ using a monolithic platform. For quantum information processing applications, this device holds great promise for phase-sensitive amplification, which is a pivotal utility in quantum key distribution systems¹⁸ and quantum metrology¹⁹ as well. Its high nonlinearity in conjunction with its ability to phase-match in the mid-IR²⁰ can lead to electrically pumped OPO chips for mid-IR spectroscopy²¹ in the future.

See the supplementary material for supporting content.