Phase-shifted Distributed Feedback Laser Research Articles

Impact of Grating Duty-Cycle Randomness on DFB Laser Performance

The duty-cycle randomness (DCR) of the Bragg grating of the distributed feedback (DFB) lasers introduced by the fabrication process is inevitable, even with state-of-the-art technologies such as electron beam lithography and dry or wet etching.This work investigates the impact of grating DCR on DFB laser performance through numerical simulations. The result reveals that such randomness causes a reduction in the side mode suppression ratio (SMSR), and deteriorates the noise characteristics, i.e., broadens the linewidth and increases the relative intensity noise (RIN). With the grating DCR, the effective grating coupling coefficient decreases as evidenced by the reduced Bragg stopband width. However, the longitudinal spatial hole burning (LSHB) effect in the DFB lasers can somewhat be diminished by the grating DCR. The seriousness of these effects depends on different grating structures and their coupling strengths. Our simulation shows that a degradation of 17 dB can be brought to the SMSR of the uniform grating DFB lasers with their duty cycles taking a deviation of ±25% in a uniformly distributed random fashion. It also broadens the linewidth of the quarter-wavelength phase-shifted DFB lasers by more than 2.5 folds. The impact of this effect on the RIN is moderate—less than 2%. All the performance deteriorations can partially be attributed to the effective reduction in the grating coupling coefficient of around 20% by such a DCR.

Phase-Shifted Distributed Feedback Lasers With Two-Layer Moiré Gratings

Single-wavelength distributed feedback (DFB) lasers are widely used in high-speed access networks; however, the device yield is generally low due to threshold degeneracy resulting from uniform periodic gratings. In this work, we propose and experimentally demonstrate a two-layer moire grating structure to provide a quarter-wavelength phase shift in DFB lasers. The grating structure can be defined easily using low-cost laser interference lithography. In experiments, 200- $\mu \text{m}$ -long DFB lasers exhibit excellent single-wavelength operating behavior (SMSR >42 dB) in terms of Bragg wavelength and high-speed modulation capability ( $\text{f}_{\mathrm {3dB}} =22$ GHz). High device yield of 86% from 320 laser dies further validated the efficacy of the proposed DFB laser design as a promising alternative to existing phase-shifted DFB laser solutions.

Experimental Demonstration of the Distributed Feedback Semiconductor Laser With S-Bent Waveguide and Sampled Grating

A distributed feedback (DFB) semiconductor laser based on s-bent waveguide and sampled grating is theoretically studied and experimentally demonstrated. The proposed laser operates with a high side mode suppression ratio (SMSR) when the bias current is changed from 50 to 130 mA. When the bias current is fixed at 130 mA, the SMSR is larger than 55 dB, which is benefit from the suppressed spatial hole burning, while the SMSR of the equivalent phase shifted DFB laser decreases to 39 dB. The wavelength spacing error between two proposed DFB lasers is measured to be 0.089 nm compared with the designed value. When temperature is increased from 20 °C to 42 °C, the presented DFB laser achieves stable single longitudinal mode (SLM) operation with SMSR >51 dB and the wavelength varies from 1562.24 to 1564.6 nm with a slope of 0.11 nm/°C. The s-bent waveguide and sampled grating can be fabricated simply by the common holography exposure and photolithograph technique resulting in a low cost. Besides, the wavelength accuracy and SLM yield can be improved significantly, which is very beneficial for the multiwavelength DFB laser array.

Study of multiwavelength DFB semiconductor laser array with asymmetric structures based on sampling technique

Multiwavelength distributed feedback (DFB) semiconductor laser arrays (MLA) with asymmetric structures are studied in this paper. Thanks to the sampling technique, the asymmetric structures, including asymmetric phase shift and asymmetric coupling coefficient, can be achieved by common holographic exposure. Therefore, the cost of fabrication is remarkably reduced. In addition, due to the large scale of the sampling pattern, the wavelength precision of these kinds of MLA can be simultaneously improved. As an example, we designed and fabricated an asymmetrically phase-shifted MLA with 10 wavelengths for the first time. Compared with the common phase-shifted DFB laser, slope efficiency is significantly improved and single longitudinal mode is still guaranteed. Besides, relatively high wavelength precision is also obtained. The proposed MLA configurations may significantly benefit multiwavelength emitters for future photonic integration.

Monolithically integrated distributed feedback laser array wavelength-selectable light sources for WDM-PON application

The monolithic integration of 1.5-µm four channels phase shift distributed feedback lasers array (DFB-LD array) with 4×1 multi-mode interference (MMI) optical combiner is demonstrated. A home developed process mainly consists of butt-joint regrowth (BJR) and simultaneous thermal and ultraviolet nanoimprint lithography (STU-NIL) is implemented to fabricate gratings and integrated devices. The threshold currents of the lasers are less than 10mA and the side mode suppression ratios (SMSR) are better than 40dB for all channels. Quasi-continuous tuning is realized over 7.5nm wavelength region with the 30°C temperature variation. The results indicate that the integration device we proposed can be used in wavelength division multiplexing passive optical networks (WDM-PON).

Phase-shifted distributed feedback laser with linearly chirped grating fabricated by reconstruction equivalent chirp technique

The phase-shifted distributed feedback (DFB) semiconductor laser with linearly chirped grating based on reconstruction equivalent chirp (REC) technique is theoretically analyzed and experimentally demonstrated for the first time. The asymmetric property of the lasing spectrum is analyzed according to the normalized threshold gain, and the different spectra from each facet of the laser are compared. Due to the low cost and fabrication flexibility, REC technique provides a promising way for the future practical applications of DFB lasers with chirped gratings.

A Multiple Phase-Shifted Distributed Feedback (DFB) Laser Fabricated by Nanoimprint Lithography

Multiple phase-shifted (MPS) diffraction grating is an effective way proposed to overcome the spatial hole burning (SHB) effect in a distributed feedback (DFB) laser. We present two symmetric λ/8 phase-shifted DFB lasers by using nanoimprint lithography (NIL). The threshold current of a typical laser is less than 15mA. The side mode suppression ratio (SMSR) is still above 42 dB even at 100 mA current injection. To show the versatility of NIL, eight different wavelength MPS-DFB lasers on this single chip are also demonstrated. Our results prove that NIL is a promising tool for fabricating high performance complex grating DFB lasers.

First demonstration of 1.3 μm quarter-wavelength shift distributed feedback (DFB) semiconductor laser based on conventional photolithography

A 1.3 μm distributed feedback (DFB) semiconductor laser with equivalent λ/4 phase shift based on reconstruction equivalent chirp (REC) technique is numerically studied and experimentally demonstrated. The simulation results show that the 1.3 μm DFB laser with equivalent λ/4 phase shift based on the REC technique performs the same as that of actual λ/4 phase shift DFB laser, with nearly the same P-I curves, the internal power distributions and the output ASE spectra. Compared with the traditional λ/4 phase shift DFB laser, the REC based laser only changes the sampling structures with the uniform seed waveguide grating instead of the actual grating structures. As a result, the fabrication of such laser will be very easy. In this paper, we successfully fabricated the 1.3 μm DFB laser based on the REC technique for the first time to the best of our knowledge.

Improved λ/4 phase-shifted DFB semiconductor laser with spatial hole burning compensation using grating chirp

A λ/4 phase-shifted distributed feedback (DFB) semiconductor laser with a preformed chirped grating used to compensate the spatial hole burning (SHB) induced index change is proposed and analyzed. It shows that compared with the three phase shifted DFB laser which is known for its good performance to eliminate SHB, the proposed SHB compensated laser has better single longitudinal mode property, narrower spectral linewidth and better dynamic characteristics.

纳米压印λ/4相移分布反馈激光器的单模稳定性

纳米压印λ/4相移分布反馈激光器的单模稳定性

Dynamic single-mode and modulation characteristics analyses for λ/4 phase-shifted distributed feedback lasers with chirped grating

The dynamic single-mode and modulation performance of λ/4 phase-shifted distributed feedback laser diode with chirped grating (QWS-CG-DFB) are analyzed theoretically. The numerical simulation shows that, in contrast to purely QWS-DFB laser, the enhanced dynamic single-mode suppression ratio (SMSR) can be reached by QWS-CG-DFB laser. Under the smaller biasing current, the modulation band-width in presence of chirped grating is narrower, this difference shrinks for larger biasing current. For large signal modulation, the chirped grating is helpful to increase the output extinction ratio, but worsens the frequency chirping. The good dynamic characteristic can be obtained by selecting system parameters reasonably.

Detailed modulation response analyses on enhanced single-mode QWS-DFB lasers with distributed coupling coefficient

By using the coupled mode equations, the dynamic characteristics of λ/4 phase-shifted distributed feedback laser diode with distributed coupling coefficient (QWS-DCC-DFB) is analyzed. The numerical simulation indicates that, compared with purely QWS-DFB laser, the better dynamic single-mode can be achieved by QWS-DCC-DFB laser; Under the smaller biasing current, the modulation band-width is narrower, this difference shrinks for larger biasing current; For large-signal modulation, the distributed coupling coefficient is helpful to increase the output extinction ratio, but worsens the wavelength chirping. The good dynamic characteristic can be obtained by selecting system parameters reasonably. QWS-DCC-DFB laser would have potentials in optics communications because of better dynamic SMSR.

Above-Threshold Spectrum of the Radiation Field in Surface-Emitting DFB Lasers

This paper addresses the above-threshold analysis of the amplified spontaneous emission spectrum of the radiation field in surface-emitting distributed feedback (DFB) lasers with a second-order grating. To the best of the authors' knowledge, this paper is the first report in this regard. The analysis takes advantage of the time- and frequency-domain approaches. The essence of this method is obtaining the spectrum of the laser power using the fields in the frequency domain, whereas the above-threshold carrier distribution inside the cavity is obtained by the finite-difference time-domain approach. The use of this approach can be justified by the fact that the wave equations in the time and frequency domains are related by the Fourier transform. Applying this method to the surface-emitting DFB lasers with a second-order grating, the authors will demonstrate that excitation of the radiation field by the interference between the counter-propagating waves inside the cavity provides additional filtering mechanism, which makes the side-mode suppression ratio of the power emitted from the surface different than that from the edge. More importantly, it is shown that the interesting features of a properly designed quarter-wave phase-shifted surface-emitting DFB lasers with a second-order grating can be exploited in the design of transmitters for optical communications

Theoretical investigation on THz wavelength conversion based on four-wave mixing in QWS-DFB-LD

By using the coupled mode equations, the characteristics of THz wavelength converter based on four-wave mixing (FWM) in λ/4 phase-shifted distributed feedback laser diode (QWS-DFB -LD) are analyzed. The numerical simulation indicates that, larger biasing current and smaller detuning expand the dynamic range of conversion efficiency and extinction ratio, but worsen the frequency chirp. Due to the lasing effect, the peak modulation frequency of extinction ratio and chirp occur at the relaxation oscillation frequency. Good conversion characteristics can be obtained by selecting system parameters reasonably. This configuration would have potentials in optics communications because of its wide-band, high-speed and without pump input.

Coherently Tunable Mid-Infrared Distributed Feedback Lasers

We propose utilization of quantum interference effects in quantum well structures to tune lasing wavelengths of mid-infrared distributed feedback lasers. The interference effects are generated via interaction of an intense laser field with an n-doped quantum well, causing coherent suppression or enhancement of refractive indexes of the conduction intersubband transitions. We show that these processes allow us to shift lasing wavelength to shorter or longer wavelengths by adjusting the intensity and frequency of the intense laser. This study is done for two types of lasers: 1) an electromagnetically induced distributed feedback intersubband laser formed by embedding a longitudinal corrugation of several periods of the quantum well structure within a waveguide structure and 2) a phase-shifted distributed feedback laser where the quantum well is inserted in the middle of an index grating, forming an active phase shift region. In the former the intense laser field is responsible for generation of optical feedback while shifting the coherently induced stop-band. In the latter, however, this field changes the optical length of the phase shift region, tuning the lasing mode within the stop-band. We show that the amount of the wavelength shift, which can reach 17 nm, is controlled by the intensity of the intense laser. The sign of the tuning process (red or blue shift), however, is decided by the frequency of this field, after proper choice of the corrugation periods. We investigate the optical feedback mechanisms in such coherently tunable lasers and discuss how they are related to an electromagnetically induced transparency process that happens in the conduction intersubband transitions.

Effects of self-mixing interference on gain-coupled distributed-feedback lasers

The effects of self-mixing interference on gain-coupled (GC) distributed-feedback (DFB) lasers are analyzed. From coupled-wave theory, the oscillation frequency and threshold gain variations are theoretically deduced. The influences on self-mixing from the coupling length coefficient, the linewidth enhancement factor of the GC DFB laser, and the reflection coefficient of the external reflector are discussed along with numerical analysis and are compared with the effects of self-mixing interference of lambda/4 phase-shifted DFB lasers and Fabry-Perot (F-P) lasers. Our results show that high-accuracy self-mixing sensors can be obtained with GC DFB lasers.

Low-chirp and external optical feedback resistant characteristics in λ/8 phase-shifted distributed-feedback laser diodes under direct modulation

Clarifies that phase shift value in directly modulated distributed feedback laser diodes (DFB-LDs) affects their chirp characteristics and resistance to external optical feedback because of the feedback effect of mirror loss (FEML). Comparing /spl lambda//8, /spl lambda//4, and 3//spl lambda/8 phase-shifted DFB-LDs, theoretically and experimentally, showed that the negative FEML in /spl lambda//8 phase-shifted DFB-LDs improves their transmission performance and makes them more resistant to external optical feedback. It was demonstrated with 2.5-Gb/s directly modulated 1.55-/spl mu/m /spl lambda//8 phase-shifted DFB-LDs that a very low-power penalty for 100-km transmissions (less than 1 dB for BER=10/sup -10/) can be obtained within a wide extinction ratio from 8.5 to 14.5 dB. Excellent resistance to optical feedback was also demonstrated using /spl lambda//8 phase-shifted DFB-LDs: isolator-free 80-km transmission was successful under -14-dB external optical feedback. In addition, the relation of transmission characteristics to coupling coefficient /spl kappa/L and detuning between oscillation wavelength and gain maximum was also discussed here for designing low chirp /spl lambda//8 phase-shifted DFB-LDs.

Analysis of Distributed Phase Shift in Distributed Feedback Semiconductor Lasers

A new algorithm based on a transfer matrix method is given to analyze a new distributed feedback (DFB) structure–distributed phase shift in DFB laser. In the small signal analysis, a new model is presented with this algorithm. Compared with λ/4 phase-shifted DFB and three phase shifted DFB lasers, this structure shows superior characteristics in both static and dynamic behavior.

Wavelength accuracy in distributed phase-shifted DFB lasers

In the wavelength-division-multiplexed transmission framework, we investigate the ability of the distributed phase-shifted distributed-feedback (DFB) structure by enlarging the ridge to fulfil the wavelength accuracy required for array fabrication. Measurements on three different phase-shift geometries indicate that the poor localization of the optical mode along the cavity is detrimental to the wavelength accuracy. This study points out the best solution for DFB structure implementable in arrays, using already validated low-cost and high-throughput technologies in a production environment.

Analysis of radiation mode effects on oscillating properties of DFB lasers

The effects of radiation mode on the oscillating properties of distributed feedback (DFB) lasers with second-order corrugations are analyzed for designing a new type of DFB laser. A formulation based on the transfer matrix technique is applied to calculating Streifer's /spl zeta/-terms added to the coupled-wave equations. These terms represent the effects of radiation and evanescent modes. This formulation greatly simplifies the analysis of distributed resonance along multilayered waveguide structures with arbitrary-shaped second-order corrugations. The effects of vertical resonance are also incorporated into the formulation. Various types of DFB lasers with phase-shifted second-order corrugations are analyzed using this method. It is found that the phase shift and the blaze of the corrugations greatly affect the longitudinal mode selectivity. A new phase-shift DFB laser structure with two complementary blazing regions connected at the shift is proposed. It is demonstrated that this structure has small radiation loss resulting in low-threshold performance despite employing second-order corrugations.