Detuned Loading Effect Research Articles

Modulation bandwidth enhancement in monolithic integrated two-section DFB lasers based on the detuned loading effect

Modulation bandwidth enhancement in a directly modulated two-section distributed feedback (TS-DFB) laser based on a detuned loading effect is investigated and experimentally demonstrated. The results show that the 3-dB bandwidth of the TS-DFB laser is increased to 17.6 GHz and that chirp parameter can be reduced to 2.24. Compared to the absence of a detuned loading effect, there is a 4.6 GHz increase and a 2.45 reduction, respectively. After transmitting a 10 Gb/s non-return-to-zero (NRZ) signal through a 5-km fiber, the modulation eye diagram still achieves a large opening. Eight-channel laser arrays with precise wavelength spacing are fabricated. Each TS-DFB laser in the array has side mode suppression ratios (SMSR) > 49.093 dB and the maximum wavelength residual < 0.316 nm.

Research on an Enhanced Detuned-Loading Effect in Integrated Two-Section DFB Lasers with High Modulation Bandwidths.

A novel high-speed directly modulated two-section distributed-feedback (TS-DFB) semiconductor laser based on the detuned-loading effect is proposed and simulated. A grating structure is designed by the reconstruction-equivalent-chirp (REC) technique. A π phase shift is introduced into the reflection grating, which can provide a narrow-band reflection region with a sharp falling slope on both sides of the reflection spectrum, thus enhancing the detuned-loading effect. Owing to its unique dual-falling-edges structure, the bandwidth can be improved even when the lasing wavelength shifts beyond the left falling edge due to a thermal effect in the actual test, in which condition the detuned-loading effect can be used twice, which greatly improves the yield. The modulation bandwidth is increased from 17.5 GHz for a single DFB laser to around 24 GHz when the lasing wavelength is located on the left falling edge of the TS-DFB laser based on the detuned-loading effect, and it can be increased to 22 GHz for the right side. An eight-channel laser array with precise wavelength spacing is investigated, with a side-mode suppression ratio (SMSR) >36 dB. In addition, TS-DFB lasers with uniform reflection gratings are studied, and simulated results show that the modulation characteristic is far inferior to the laser with a phase-shifted grating reflector.

High-speed directly modulated distributed feedback laser based on detuned loading and photon–photon resonance effect

A monolithic integrated two-section distributed feedback (TS-DFB) semiconductor laser for high-speed direct modulation is proposed and analyzed theoretically. The grating structure of the TS-DFB laser is designed by the reconstruction-equivalent-chirp (REC) technique, which can reduce the manufacturing cost and difficulty, and achieve high wavelength controlling accuracy. The detuned loading effect and the photon–photon resonance (PPR) effect are utilized to enhance the modulation bandwidth of the TS-DFB laser, exceeding 37 GHz, while that of the conventional one-section DFB laser is only 16 GHz. When the bit rate of the non-return-to-zero (NRZ) signal reaches 55 Gb/s, a clear eye diagram with large opening can still be obtained. These results show that the proposed method can enhance the modulation bandwidth of DFB laser significantly.

Modulation Bandwidth Enhancement and Frequency Chirp Suppression in Two-Section DFB Laser

We demonstrate a two-section distributed feedback (DFB) laser for direct modulation based on the reconstruction equivalent chirp (REC) technology. The two-section DFB laser exhibits a stable single-mode characteristic. Utilizing the photon-photon resonance (PPR) effect and detuned-loading effect, the 3-dB modulation bandwidth of the two-section DFB laser is improved from 8 GHz to 23 GHz. A 25 Gb/s non-return to zero (NRZ) signal is applied to the two-section DFB laser and eye diagrams with large open extent are obtained. In addition, the relative intensity noise is reduced to below −143 dB/Hz. The frequency chirp is also suppressed. The chirp parameter is reduced from 5.67 to 1.37, which is a small value in directly modulated DFB laser. We compared the eye diagrams after transmitting different distances, the results show that the two-section structure reduces the frequency chirp significantly. The modulation of the two-section DFB laser also has good linearity, with a 1-dB compression point of more than 18 dBm.

50-GHz Repetition Gain Switching Using a Cavity-Enhanced DFB Laser Assisted by Optical Injection Locking

We demonstrate pulse generation at a repetition rate of 50 GHz by gain switching an injection-locked distributed feedback (DFB) laser. The small-signal BW of the DFB laser was enhanced from an intrinsic BW of 30 GHz to 52 GHz due to the joint effects of photon-photon resonance (PPR) and detuned loading. The detuned loading effect is achieved by exploiting the frequency-dependent cavity loss, which results in an increase of the effective differential gain, and correspondingly, the relaxation oscillation frequency. In addition to small-signal measurements, we show that the PPR and the detuned loading effects can also be utilized to improve large signal gain-switching, despite the dynamic changes of the detuned loading condition due to the large signal chirp. By modulating the laser with 50-GHz RF signals, we obtained 50-GHz repetition rate pulses from the gain-switched laser, confirming that the BW enhancement effects are still valid for large-signal modulation. Subsequently, we optically injection lock the gain-switched laser with strong external seeding light, which suppresses the chirp and creates coherent frequency tones with 50 GHz spacing. The optical injection locking (OIL) further enhances the large-signal BW, resulting in narrower pulse width of 9.5 ps (6.2 ps after deconvolution) in the time domain and three sideband peaks above 70% from the peak in the frequency domain (full width half maximum of 130 GHz). Assuming linear chirp, the pulse could be further compressed to 2.4 ps.

A Chirp-Managed Tunable DBR Laser Based on Novel Cascaded Gratings

In this paper, a novel tunable DBR laser based on novel cascaded gratings is proposed for chirp management in high-speed direct modulation applications. The cascaded gratings, i.e., a tilted π phase-shift grating followed by a normal uniform Bragg grating, can provide a narrow-band reflection spectrum with a sharp falling slope on the longer wavelength side, so that the optical spectrum shaping and the detuned-loading effects can be enhanced. Simulation results indicate that the wavelength tuning range of the laser is about 10 nm and the 3-dB modulation bandwidth reaches up to 25 GHz. By simply adjusting the phase current and thus changing the detuned frequency, the extinction ratio and the frequency chirp can be optimized, which is helpful to improve the dispersion tolerance and reduce the dispersion power penalty.

Modulation Bandwidth Enhancement in Distributed Reflector Laser Based on Identical Active Layer Approach

We demonstrate a distributed reflector (DR) laser with the distributed feedback (DFB) section and the distributed Bragg reflector (DBR) section sharing the same multiple-quantum-well structure. The DR laser exhibits a stable dynamic single-longitudinal mode characteristic. A direct modulation bandwidth of 27 GHz is obtained through the detuned-loading effect and photon–photon resonance effect. The fabrication process of the DR laser is simplified due to the identical active layer structure and the uniform grating in the DFB and DBR sections.

Investigation Into the Phase Noise of Modulated Grating Y-Branch Lasers

Theoretical and experimental investigations of the phase noise of modulated grating Y-branch (MGY) lasers are presented, mainly focusing on influence of the additive Vernier effect. A comprehensive simulation model based on transmission line model is developed, taking into account the spontaneous emission noise in the active section, the carrier shot noise in the passive sections, the 1/f noise for the injection currents and the detuned-loading effect induced by the grating reflectors. The phase noise characteristics of the MGY laser and the SGDBR laser are simulated, and compared to find the difference between the additive and multiplicative Vernier effects. The results clearly indicate that the MGY laser exhibits more serious phase noise due to using the additive Vernier effect. It has been found that different super-mode selection methods lead to different reflection characteristics, which play an important role in determining the phase noise. Finally, the phase noise of the MGY laser and the SGDBR laser are measured by using a coherent phase noise measurement technique, the experimental results turn out to be in agreement with the simulations.

55 GHz Bandwidth Distributed Reflector Laser

We have demonstrated a 1300-nm short-cavity distributed reflector (DR) laser having 55 GHz bandwidth (BW), and successful 112-Gb/s transmission using four-level pulse-amplitude-modulation (PAM-4), without any pre-equalization for the transmitter. Two effects were realized in the design and operation of the DR laser: photon–photon ( P–P ) resonance and detuned-loading. The P–P resonance effect was realized between the DFB and distributed Bragg reflector (DBR) modes that coexisted in the cavity of the DR laser. The detuned-loading effect was used to effectively enhance the differential gain through the dynamic change in the mirror reflectivity that occurs on the flank of the DBR mirror due to the frequency chirp under modulation. Despite a limited RC cutoff frequency of 22 GHz, a wide modulation BW of 55 GHz was achieved. It is shown that the RC limitation was counteracted by the combined effects of the detuned-loading, which reduces the damping of relaxation oscillations, and an in-cavity FM–AM conversion effect that created a high-pass filter effect in the modulation response. This will be discussed by way of simulations and experimentally observed eye diagrams for 10 Gb/s NRZ and 28 Gbd PAM-4. The short-cavity DR laser achieved 112-Gb/s PAM-4 transmission over links having a range of dispersions from –28 to +7 ps/nm without precompensation on the transmitter.

Simple detuning method for low-chirp operation in polymer-based tunable external-cavity lasers.

We propose and demonstrate a simple detuning method for the low-chirp operation of a polymer-based tunable external-cavity laser (ECL). To ensure the low-chirp operation of this directly-modulated ECL, we first obtain the optimum values of the heater current applied to the polymer Bragg grating reflector (PBR) and the operating temperature of this ECL. For this purpose, we sweep the current applied to the phase control heater until the peak output power measured from the high-reflection (HR) coated facet reaches the minimum value. We then operate this ECL with minimum chirp by tuning the lasing mode to the longer wavelength limit of the stable operation region. This is because the detuned loading effect is maximized at this limit as the in-phase condition between the lights reflected from the PBR and anti-reflection (AR) coated facet of the gain medium is satisfied. Thus, by using this method together with conventional wavelength-locking algorithm, we can operate this ECL with minimum chirp at any wavelength.

Evaluation of Chirp Reduction in Polymer-Based Tunable External-Cavity Lasers

We evaluate the chirp reduction achievable using the detuned loading effect in the polymer-based tunable externalcavity laser (ECL) capable of operating at 10 Gb/s. In particular, we estimate the achievable linewidth enhancement factor (LEF) of this ECL considering the stable operation region increased by the nonlinear gain. The results show that, due to the nonlinear gain, we can detune the lasing mode further away from the nominal detuning limit (i.e., half of the mode spacing) by ~4 GHz for an ECL with a mode spacing of ~30 GHz under the continuous-wave (CW) operating condition. As a result of this additional detuning, the effective LEF is reduced from 1.3 to 0.8, although its material LEF is ~5. However, when we change the operating wavelength of this ECL, the minimum achievable LEF is deteriorated due to the parasitic reflection occurring at the antireflection (AR)-coated facet. For example, when the optical power reflected at the AR-coated facet is ~5% of the feedback power from the polymer Bragg grating reflector (PBR), the effective LEF is increased from 0.8 to 3.4. To verify these results experimentally, we fabricate a polymerbased tunable ECL and measure its chirp characteristics. For example, we directly modulate this ECL at 10 Gb/s and measure its effective LEF as a function of the detuned frequency. The result shows that we can reduce the effective LEF to ~1 by detuning the operating mode by 18 GHz. This value is slightly larger than that achievable under the nearly CW operating condition (i.e., 0.8) due to the chirp-induced mode hopping. We also confirm that, by optimizing the detuning condition, this ECL can be used for the transmission of a 10-Gb/s signal over 20 km of a standard single-mode fiber (SSMF) with a power penalty of <;2 dB. However, an error-free transmission cannot be achieved at some wavelengths due to the parasitic reflection. To avoid this problem, it is needed to suppress the parasitic reflection to be <;0.1% of the feedback power from the PBR.

Detuned loading effect and high-speed modulation of fiber grating semiconductor lasers

We have studied both theoretically and experimentally the detuned-loading effect of a fiber grating semiconductor laser. This effect is observed experimentally with a fiber grating semiconductor laser constructed with a single-angled facet diode and a conically lensed grating. Operation of such a laser is demonstrated at 2.5 Gb/s and good eye diagrams are obtained.

Short cavity distributed Bragg reflection lasers with a narrow band Bragg mirror: Dynamic characteristics

The dynamical characteristics of distributed Bragg reflection lasers with a Bragg reflection bandwidth narrower than the cavity mode spacing are reported. The operation of this Bragg reflector as an amplitude modulator (unlike the usual frequency modulator) is demonstrated in the small and large signal modulation regimes. In the conventional mode of gain modulation, this narrow Bragg reflector exhibits detuned loading effects. The laser is capable of efficient digital modulation as well as Q-switching, both with a very small drive current.

Linewidth and alpha-factor of detuned-loaded DBR lasers

The authors report measurements of the alpha -factor and linewidth of DBR (distributed Bragg reflector) lasers fabricated with weak grating constants. Such devices exhibit detuned-loading effects, and the measured values depend strongly on the tuning current of the Bragg section. alpha -factor behavior is found to agree with earlier theoretical predictions. Linewidth behavior is shown to be dominated by carrier shot noise in the tuning section. A theoretical analysis of the DBR laser is outlined including detuned loading and carrier shot noise effect. >