Modulated DFB Research Articles

Ultralong \u03c0-phase shift fiber Bragg grating empowered single-longitudinal mode DFB phosphate fiber laser with low-threshold and high-efficiency

Phosphate glass fiber is one of the candidates for building compact fiber lasers because of its capability of high-concentration of rare-earth ions doping in fiber core. Nevertheless, it is challenging for the integration of UV-written intra-core fiber Bragg gratings into the fiber laser cavity due to the low photosensitivity of phosphate glass fiber. The research presented in this paper will focus on demonstration of UV-written Bragg gratings in phosphate glass fiber and its application in direct-written short monolithic single-frequency fiber lasers. A 5-cm-long strong π-phase shift Bragg grating structure is direct-inscribed into the Er/Yb co-doped gain fiber using an excimer laser. The fiber laser device emits output power of 10.44 mW with a slope efficiency of 21.5% and the threshold power is about 42.8 mW. Single-longitudinal mode operation is validated by radio frequency spectrum measurement. Moreover, the output spectrum at the highest power shows an excellent optical signal to noise ratio of about 70 dB. These results, to the best of our knowledge, show the lowest power threshold and highest efficiency among the reports that using the same structure to achieve single-longitudinal mode laser output.

Experimental demonstration of a hybrid integrated dual-parallel modulated DFB semiconductor laser with suppressed nonlinear distortions

A hybrid integrated dual-parallel modulated DFB laser is experimentally demonstrated. Utilizing the proposed method, the nonlinear distortions of the DFB laser, including the second harmonic distortion (2HD) and the third-order intermodulation distortion are suppressed significantly. Comparing with the single modulated DFB laser, the 2HD is reduced by 8.6 dB and the spurious free dynamic range is increased from 87.2 dB·Hz2/3 to 90.3 dB·Hz2/3.

Laterally coupled distributed feedback type-I quantum well cascade diode lasers emitting near 322 μm

The laterally coupled distributed feedback (LC-DFB) GaSb-based type-I quantum well cascade diode lasers using the second- and the sixth-order gratings to stabilize the output spectrum near 3.22μm were designed and fabricated. The laser heterostructure contained three cascades. The devices were manufactured using a single dry etching step defining the ∼5-μm-wide ridge with ∼5-μm-wide gratings sections adjacent to the ridge sides. The grating coupling coefficients were estimated to be about 1 cm-1. The stability of the single-frequency operation was ensured by alignment of the DFB mode to the relatively wide gain peak. The 2-mm-long second-order LC-DFB lasers generated above 10mW of continuous-wave (CW) output power at 20°C in epi-side-up configuration and demonstrated power conversion efficiency above 2%. The sixth-order LC-DFB lasers showed lower efficiency but still generated several milliwatts of CW output power. The devices demonstrated a CW current tuning range of about 3.5nm at the temperature of 20°C.

1 Gbps full-duplex links for ultra-dense-WDM 6.25 GHz frequency slots in optical metro-access networks.

1 Gbps full-duplex optical links for 6.25 GHz ultra dense WDM frequency slots are demonstrated and optimized for cost-effective metro-access networks. The OLT-ONU downlinks are based on 1 Gbps Nyquist-DPSK using MZM and single-detector heterodyne reception obtaining a sensitivity of -52 dBm. The ONU-OLT uplinks are based on 1 Gbps NRZ-DPSK by directly phase modulated DFB and also single-detector heterodyne reception obtaining same sensitivity of -52 dBm. The power budget of full-duplex link is 43 dB. These proposed links can provide service to 16 (32) users at each 100 (200) GHz WDM channel.

High-Power Tunable Dilute Mode DFB Laser With Low RIN and Narrow Linewidth

We have developed a 1-mm-long high-power DFB laser using an asymmetrical cladding based on the dilute waveguide technique. We have obtained about 180 mW output power at 25°C and >;30 mW at 15°C-85°C chip temperature with >;55-dB sidemode suppression ratio. This temperature range allows a 9.7-nm wavelength tunability. For high output power, the relative intensity noise is lower than -160 dB/Hz in the 0.08-40-GHz frequency range and the optical linewidth is better than 300 kHz.

변조된 DFB 레이저를 이용한 광 송수신기의 SMSR 수율 향상에 관한 연구

본 논문은 광 통신용 송수신기의 핵심 부품인 DFB 레이저의 SMSR 수율 향상에 관한 연구이다. 광 송수신기는 주로 1310, 1490 및 1550 nm 파장의 DFB 레이저를 사용하고 있는데, 변조된 DFB 레이저의 SMSR 특성에 따라 전송효율이 결정되므로, 변조된 DFB 레이저의 SMSR 특성을 확보하는 것이 매우 중요하다. 하지만, 대량 생산시변조된 상태의 광 송수신기 제품이 SMSR 사양에 만족하기가 어려울 경우, 불량률이 높아지는데, 불량률을 감소시키기 위해 광 송수신기의 광 파워 출력과 소광비를 조정하여 SMSR을 개선할 수 있는 연구를 하였고, 이를 실제 적용하여 SMSR 불량률을 감소시켰다. This paper is the research on SMSR yield improvement of the optical transceiver using modulated DFB laser. In general, the wavelength of DFB laser optical transceiver are 1310, 1490 and 1550 nm. Usually SMSR in modulated DFB is difficult to improve as low as 30 dB because of high slop efficiency trade off. In modulation condition, we studied SMSR improvement according to adjust bias current, extinction ratio and optical line terminal receiver sensitivity. As our test results, we can found a method how to improve SMSR for optical transceiver for long distance.

Elimination of low-frequency fluctuations of backscattered Rayleigh radiation from optical fiber with chaotic lasers

We have experimentally demonstrated suppression of low-frequency fluctuations of backscattered Rayleigh radiation from optical fiber with chaotic single-longitudinal mode DFB and multi-longitudinal mode FP lasers subjected by incoherent optical feedback. Significant decreasing of Rayleigh power variations up to 15–20 dB for 10–1000 Hz frequency interval was recorded for both chaotic lasers. It was shown that chaotic DFB laser also efficiently suppresses stimulated Brillouin scattering in the test fiber.

Experimental characterization of DFB and FP chaotic lasers with strong incoherent optical feedback

We have experimentally investigated chaotic power oscillations in single-longitudinal mode DFB and multi-longitudinal mode FP lasers as a function of incoherent optical feedback strengths of up to 42%. We have demonstrated the existence of chaos in the output oscillations of both laser types using classical experimental tools such as RF spectrum, standard deviation, and maximum Lyapunov exponent, which all increase with increasing of feedback strength for both in single-longitudinal mode DFB lasers and multi-longitudinal mode FP lasers. It is also shown that power switching among longitudinal modes of multimode FP semiconductor laser is a considerable portion of the chaotic power oscillations for both strong and weak incoherent optical feedback.

Study of dynamic chirp in direct modulated DFB laser for C-OFDR application

A description of the chirp induced by direct modulated DFB laser is presented. Two approaches are considered: the first one is based on a resolution of laser rate equations; the second, on a simulation with a commercial software. We compare results of the two approaches, we demonstrate that the optical frequency can be controlled via the injected current. We also characterize the linear variation of the optical frequency in time (for triangular and sawtooth modulation), in order to choose the appropriate values of laser and modulation parameters for a perfect linearity of the chirp in time. This study will be very helpful to validate the use of direct linear modulated DFB laser as a tunable source in Coherent Optical Frequency Domain Reflectometry technique C-OFDR. We present a simulation result of Mach–Zehnder delay time measurement based on C-OFDR system using a direct modulated semiconductor laser source. The obtained results are very important because it depicts the beat frequencies relating to each delay time, with respect to the modulation format used. This is very encouraging for the implementation of an experimental C-OFDR.

Experimental evidence for the thermal origin of1/ffrequency noise in erbium-doped fiber lasers

We present experimental evidence in support of the recent theoretical proposal that intrinsic $1/f$ frequency noise in short cavity erbium-doped fiber lasers is of thermal origin. We demonstrate that the power spectral density of frequency noise in distributed-feedback fiber lasers (DFB FL) exhibits predicted ${T}^{2}$ temperature dependence across all frequencies over a temperature range of almost 200 K. This temperature dependence is observed both in direct interferometric measurements of frequency noise in a single mode DFB FL and noninterferometric measurements of polarization-beat-frequency noise in a dual frequency DFB FL. It is also shown that frequency noise of orthogonal polarization modes in the dual frequency DFB FL is substantially correlated providing a strong indication of a common origin.

Characterization of the chirp in semiconductor laser under modulation

In this paper we present, shortly, a study of the chirp induced in direct modulated DFB laser. Our description of the chirp is based, first on analytical resolution of the rate equations, and second on a simulation of a heterodyne system with the Optisystem software. This study enables us to visualize the chirp of a DFB laser in the RF field and to characterize it according to the parameters of the modulation of the injection current i(t). We show by using the powerful Volterra series theory that the linear sweep of the laser optical frequency can be obtained by a triangular modulation of the injection current.

Injection locked multi-section gain-coupled dual mode DFB laser for terahertz generation

A dual mode multi-section gain-coupled distributed feedback laser with tunable mode spacing is subharmonically injection locked at 0.315 THz. The injected signal consists of an optical comb with harmonics 35 GHz apart and a bandwidth of approximately 1.9 THz. The optical comb is a result of strong four-wave mixing in a highly-nonlinear dispersion-shifted fiber. In order to observe locking of the multi-section laser, the output is optically downconverted to RF frequencies using the same optical comb. The locked multi-section DFB laser is a coherent and tunable optical source suitable for continuous-wave terahertz generation systems.

Low threshold distribution feedback quantum cascade lasers at 7.6 μm grown by gas source molecular beam epitaxy

We have demonstrated the first Gas source molecular beam epitaxy (GSMBE) grown distributed feedback quantum cascade laser (DFB QC-laser) laser with low threshold current density operation at 7.6 μm. High quality DFB QC-laser heterostructures were grown by an one step GSMBE growth procedure. The first-order grating was defined by a holographic technology. A tunable single mode DFB QC-laser operation in pulsed mode with a low threshold current density of 574 A/cm 2 and a peak optical power of 50 mW at 70 K and 7.6 μm have been obtained. The side mode suppression ratio and the maximum single mode wavelength tuning range by changing their heat sink temperature are 23 dB and 20 nm, respectively.

Laterally Coupled InGaN/GaN DFB Laser Diodes

For group III nitride-based lasers a new approach to realise DFB lasers and arrays is presented. Laterally coupled DFB (LC-DFB) lasers are realised to minimise processing-induced damage and to enable post-processing adjustment of DFB laser parameters according to the material parameters achieved after epitaxial growth. These DFB laser arrays enable one to measure the threshold and the refractive index of electrically pumped DFB lasers over a wavelength emission range of 6 nm. DFB emission is demonstrated up to 70 °C and simultaneously a small temperature shift of the DFB mode. A significantly smaller tuning range is observed compared to optically pumped DFB lasers as well as a significantly red-shifted gain region. Furthermore, the dispersion relation gradient determined, dn eff /dλ, appears significantly steeper (by a factor of six) than in the optically pumped case. These effects are attributed to incompletely screened polarisation fields in electrically pumped InGaN/GaN lasers.

Wide temperature range operation of1.55 µm current-blocking gratingcomplex-coupled DFB laser

The authors demonstrate, for the first time, good quality transmission of a 2.488 Gbit/s signal by a 1.55 µm complex-coupled DFB laser at 85°C. The laser oscillates in a DFB mode with a side-mode suppression ratio of greater than 43 dB, the slope efficiency varies from 0.36 to 0.22 in the temperature range 20 – 90°C; 5 mW is obtained at 90°C with a driving current of < 50 mA.

Noise Analysis in Coherence-Multiplexed Optical Fiber Communication Systems

By exploiting the coherence properties of optical sources, many users can be multiplexed on to a single channel achieving a secure, asynchronous mode of transmission, free of central network control. In this paper an analysis of the impact of interference from unwanted users is described and limitations imposed by shot, phase, and partition noise on coherence multiplexed system performance employing LED, multimode laser diode, DFB, and single mode Fabry–Perot laser diode sources are considered and compared. The determination of the amount of delay required to compensate for system limitation imposed by phase noise arising due to mismatches between transmitter and receiver interferometers to achieve a 10−9BER and the impact of interferometric noise in the perfectly matched scenario are presented.

Improved resonance and chirp mechanisms for multigigabit/s push–pull modulated DFB lasers

The internal resonance within a push-pull modulated DFB laser is investigated with numerical and analytical analysis. The classic photon-electron resonance is reduced in amplitude, and the operation of the laser is dominated by a new resonance which is dependant on the mode spacing between the main lasing mode and its nearest side mode. Simulations show that this allows modulation bandwidths well beyond the conventional photon-electron resonance limit, The interaction between the dominant modal resonance and the weaker photon-electron resonance leads to a novel time resolved chirp shape which improves the properties for transmission of a pulse along an optical fibre. This chirp shape can be tailored to further improve transmission properties by controlling the interaction between the resonances. Simulations of the time resolved chirp support published experimental results.

Impedance-matched optical link for C-band satellite applications

Optical transmitter and receiver modules with passive impedance-matching circuits have been designed, constructed, and tested. A direct current modulated InGaAs DFB laser, operating at 1.3 micron, and an InGaAs PIN photodiode were matched to 50 ohms with passive, mixed lumped and distributed element, matching circuits. A link-insertion loss of 21 dB with a 3 dB bandwidth of 900 MHz has been demonstrated. Through the use of higher-order matching circuits, link-insertion loss variations across the satellite downlink frequency band (3.6-4.2 GHz) have been kept below /spl plusmn/0.5 dB. >

Theory of enhanced amplitude modulation bandwidth in push-pull modulated DFB lasers

Push-pull modulation of a two contact, uniform DFB laser enables amplitude modulation bandwidths well beyond the conventional electron-photon resonance limit for single contact devices. A small signal analytic theory for this AM response is given. >

Long-wavelength InGaAsP/InP multiquantum well distributed feedback and distributed Bragg reflector lasers grown by chemical beam epitaxy

We demonstrated the successful operation of long-wavelength InGaAsP low threshold-current index-coupled and gain-coupled DFB lasers grown by chemical beam epitaxy (CBE). For index-coupled DFB lasers, buried-heterostructure six-QW DFB lasers (250 /spl mu/m long and as-cleaved) operated at 1.55 /spl mu/m with CW threshold currents 10-15 mA and slope efficiencies up to 0.35 mW/mA (both facets). A side-mode suppression ratio (SMSR) as high as 49 dB was obtained. The lasers operated in the same range even at high temperatures (70/spl deg/C checked). For gain-coupled DFB lasers, gain-coupling is accomplished by using a InGaAsP quaternary grating or quantum-well grating that absorbs the DFB emission. The use of a quantum-well grating, in particular, greatly facilitates the reproducible regrowth (defect-free) over grating and the control of the coupling coefficient. CW threshold currents mere in the range of 10-15 mA for 250-/spl mu/m and 13-18 mA for 250-/spl mu/m and 500-/spl mu/m cavities, respectively. Slope efficiencies were high, /spl sim/0.4 mW/mA (both facets). SMSR was as high as 52 dB and remained in the same DFB mode with SMSR staying /spl sim/50 dB throughout the entire current range. Linewidth/spl times/power products of 1.9-4.0 mere measured with minimum linewidths of 1.8-2.2 MHz. No detectable chirp was measured under 2.5 Gb/s modulation. Unlike index-coupled DFB lasers in which mode partition events decrease slowly even when biased above threshold, these lasers have mode partition events shut off sharply as bias approaches threshold (/spl gsim/0.95 I/sub th/). A very small dispersion penalty of 1.0 dB was measured at 10/sup -11/ BER in transmission experiments using these lasers as sources at 1.7 Gb/s over an amplified fiber system of 230 km. No self-pulsation was observed in these gain-coupled DFB lasers. Gain-switching at 4 GHz with a 100% optical modulation depth and a FWHM pulse width of 23 ps was achieved with these gain-coupled DFB lasers. The peak power was /spl sim/72 mW and the FWHM bandwidth was 0.14 nm. We also fabricated InGaAs/InGaAsP multiquantum-well DBR lasers by CBE. Taking advantage of uniform thickness growth and proper design of weak and long gratings, a record high SMSR of 58.5 dB was obtained. >