Makoto Shimokozono

Low-driving-voltage electro-optic modulator with novel KTa1-xNbxO3 crystal waveguides

We have successfully demonstrated a low-driving-voltage electro-optic (EO) modulator using newly developed KTa1-xNbxO3 (KTN) buried waveguides. We prepared high-quality and large KTN crystals. The crystals exhibited a large quadratic EO coefficient of 4.8×10-15 m2/V2 at 1.55 µm. The KTN crystals also exhibited a very large linear EO effect, for example 600 pm/V at a biased potential of 60 V/mm, which is twenty times larger than the r33 of LiNbO3. The waveguide layers for the core and cladding were formed by liquid phase epitaxy techniques on the KTN crystals. The propagation loss was less than 0.5 dB/cm and the polarization dependent loss was less than 0.1 dB/cm. The EO modulators had a low Vπ of <2.5 V (4.0-V biased potential, l=6 mm) and a fast response of 3 GHz.

2-µm wavelength tunable distributed Bragg reflector laser

We demonstrate a tunable distributed Bragg reflector (DBR) laser diode operating in the 2-μm region. A strained InGaAs multi-quantum well is used for the active region, and InGaAs is used for the passive region in the 2-μm DBR laser. The fabricated DBR laser emits a single-mode continuous wave at 2.02μm, and the output power exceeds 6mW at room temperature. A wavelength tuning range of about 10 nm is achieved by controlling of injection currents to the DBR regions.

Stable wavelength switching within 50 ns using tunable distributed amplification (TDA-) DFB lasers

Aiming fast wavelength switching for the tunable distributed amplification (TDA-) DFB lasers, the transfer function is deduced from the transient response and then the feedforward controller is designed. Experimental results show that the wavelength can be accurately stabilized within 35 ns and 30 ns when the optical frequency changes from 192.8 THz to 193 THz and vice versa, respectively.

Fast wavelength switching with tunable distributed amplification distributed feedback laser by feedforward control technique

We proposed a method of accelerating the wavelength switching of a tunable distributed amplification distributed feedback (DFB) laser by feedforward control. From experimental laser responses, we designed and simulated feedforward controllers using MATLAB/Simulink so as to reduce the time for wavelength stabilization. We applied this method to two different cases involving a laser drive circuit with a large output current and a 50 ns rise time, and a laser drive circuit with a moderate output current and a 5 ns rise time. In both cases, we showed that the wavelength switching times were improved from 450 to 150 ns and from 75 to 35 ns, respectively. From these results, the proposed control technique is confirmed to be efficient for possible practical applications.

35 ns wavelength switching with +/-1 GHz wavelength accuracy using tunable distributed amplification (TDA-) DFB lasers

For fast wavelength switching of TDA-DFB lasers, the feedforward controller is designed. Experimental results show switching times of 35 ns and 30 ns when the optical frequency changes from 192.8 THz to 193.0 THz and vice versa.

Sub-microsecond wavelength stabilization of tunable lasers with the internal wavelength locker

We proposed a method of accelerating the wavelength stabilization after wavelength switching of the tunable distributed amplification-distributed feedback (TDA-DFB) laser using the internal wavelength locker to reduce the size and the cost of the wavelength control system. The configuration of the wavelength stabilization system based on this locker was as follows. At the wavelength locker, the light intensity after an optical filter is detected as a current by the photodiodes (PDs). Then, for estimating the wavelength, the current is processed by the current/voltage-converting circuit (IVC), logarithm amplifier (Log Amp) and field programmable gate array (FPGA). Finally, the laser current is tuned to the desired wavelength with reference to the estimated wavelength. With this control system the wavelength is stabilized within 800 ns after wavelength switching, which is even faster than that with the conventional control system.

Fast wavelength stabilization of tunable lasers with the internal wavelength locker

The wavelength control system with the internal wavelength locker of the tunable lasers is configured so as to accelerate the wavelength detection speed. With this control system the wavelength is stabilized within 3.8 μs after wavelength switching which is even faster than that with the conventional control system.