Shigeru Mieda

Harmonic superposition for tailored optical frequency comb generation by a Mach–Zehnder modulator

This Letter demonstrates tailored optical frequency comb (OFC) generation using a LiNbO3 Mach-Zehnder modulator driven by a combination of first- and second-order harmonics of the RF signal. A quasi-rectangular-shaped OFC with less than 1 dB flatness among 11 lines was experimentally obtained when a slight second-order harmonic of the RF signal (0.1 times the half-wavelength voltage) was introduced. Good agreement was obtained between the measured and calculation results for OFCs. We discuss conditions to obtain flat OFCs using this method along with details concerning OFC conversion efficiency and bandwidth.

High-Bandwidth Operation of Optically Controlled Semiconductor Laser With External Cavity

A high-speed optically controlled semiconductor laser was numerically and experimentally demonstrated by integrating a phase controllable external cavity with a single mode semiconductor laser. It was numerically confirmed that the laser source can be operated at up to 50 GHz when the lasing-mode gain was directly controlled by an optical signal. The optically controlled semiconductor laser, which has an optimally designed structure using the results from the numerical analysis, was fabricated. A broad 3-dB bandwidth of >30 GHz was experimentally confirmed, which was the limit of the measurement system bandwidth of the experimental setup.

Ultrahigh-speed operation of laser diode by cross-gain modulation using external cavity

An optically controlled laser diode with an intensity- and phase-controllable external cavity was studied in order to achieve an increased bandwidth. The external cavity functions as the feedback section for the laser diode, using time-delayed laser light. A 3-dB bandwidth of over 70 GHz was numerically predicted, using rate equations that include cross-gain modulation and multiple optical feedback from the external cavity. A 3-dB bandwidth of more than 40 GHz was experimentally achieved with a fabricated device.

Intra-cavity loss modulation for ultrahigh-speed direct modulation lasers based on photon–photon resonance

The characteristics of the intra-cavity loss modulation laser diode (ICLM-LD) proposed in this paper were numerically and experimentally evaluated. A lower modulation sensitivity degradation rate (MSDR) of −0.30 dB/GHz and a wider 3 dB bandwidth of 49 GHz compared to those of direct modulation lasers (−0.49 dB/Hz, 16 GHz) were numerically confirmed. In addition, an MSDR of −0.29 dB/GHz and a 3 dB bandwidth wider than 30 GHz were experimentally confirmed using the fabricated ICLM-LD. The ICLM-LD will thus become a key part of ultrahigh-speed direct modulation lasers with external cavities whose modulation sensitivity is enhanced by the photon–photon resonance effect.

Ultra-Wide-Bandwidth Optically Controlled DFB Laser With External Cavity

Modulation bandwidth enhancement of an optically controlled single-mode semiconductor laser is achieved by introducing an external cavity into the laser. A calculation based on a rate equation model shows that the 3-dB bandwidth of the optically controlled external cavity laser (OC-ECL) can be substantially enhanced by controlling the feedback light intensity and phase properly. A broad 3-dB bandwidth of 59 GHz is experimentally confirmed by using the fabricated OC-ECL. The results suggest that a combination of cross-gain modulation and photon-photon resonance is suitable to enhance the modulation bandwidth of semiconductor lasers.

Transmission performance improvement of semiconductor lasers by hybrid modulation scheme

We propose a hybrid modulation scheme which has a wider 3-dB bandwidth and higher dispersion tolerance compared with a direct current modulation (DM) scheme. Frequency responses and eye diagrams are calculated by using rate equations. The calculated frequency response shows that the hybrid modulation scheme improves modulation sensitivity degradation at a high-frequency region and enhances 3-dB bandwidth of a semiconductor laser. Comparison of the eye diagrams between DM and hybrid modulation schemes shows that the hybrid modulation scheme has a good dispersion tolerance compared with the DM scheme thanks to its negative chirp property.

Tailoring of optical frequency comb shape by harmonic signal superposition in Mach-Zehnder modulator

We demonstrate tailored optical frequency comb generation by using a LiNbO3 Mach-Zehnder modulator driven by a combination of fundamental modulation RF signal and its second-order harmonic. A quasi-rectangular-shaped optical frequency comb having less than 0.5-dB flatness among 11 lines is numerically and experimentally obtained with a slight second-order harmonic superposition on the RF signal. In this method, we discuss an optimum condition of the superposed harmonic signal for obtaining flat optical frequency combs with bandwidth tunability.

Frequency response control of semiconductor laser by using hybrid modulation scheme

A hybrid modulation scheme that simultaneously applies the direct current modulation and intra-cavity loss modulation to a semiconductor laser is proposed. Both numerical calculations using rate equations and experiments using a fabricated laser show that the hybrid modulation scheme can control the frequency response of the laser by changing a modulation ratio and time delay between the two modulations. The modulation ratio and time delay provide the degree of signal mixing of the two modulations and an optimum condition is found when a non-flat frequency response for the intra-cavity loss modulation is compensated by that for the direct current modulation. We experimentally confirm a 8.64-dB improvement of the modulation sensitivity at 20 GHz compared with the pure direct current modulation with a 0.7-dB relaxation oscillation peak.

Gently-sloped small signal response by intra-cavity loss modulation

An intra-cavity loss modulation laser diode (ICLM-LD) was proposed and its characteristics were evaluated numerically and experimentally to realize an ultra wideband direct modulation external cavity laser. It was confirmed that the intra-cavity loss modulation can suppress the modulation sensitivity degradation over the relaxation oscillation frequency, which will be effective to widen the modulation bandwidth of external cavity assisted semiconductor laser drastically. Thanks to the suppression of the modulation sensitivity degradation, wide 3-dB bandwidth of more than 30 GHz was confirmed by the ICLM-LD.

High-speed optically controlled semiconductor light source with an external cavity

An optically controlled semiconductor laser with broad 3-dB bandwidth is achieved by introducing a phase- and intensity-controllable external cavity into a DFB laser. The external cavity functions as a passive-feedback section for the semiconductor laser. The optimum structure of an optically controlled passive-feedback laser (OC-PFL) for widening the 3-dB bandwidth of the laser is numerically analyzed. A broad 3-dB bandwidth of up to 40 GHz is experimentally confirmed by using a fabricated OC-PFL with a structure designed on the basis of the analysis results. It is also experimentally confirmed that it is possible to achieve 3-dB bandwidth of more than 50 GHz.