CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2020-006613, filed on Jan. 20, 2020, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an optical modulator and a control method of the optical modulator.

BACKGROUND

An optical modulator is one of key devices for attaining speed-enhancement for an optical transmission system or increasing the capacity thereof. In particular, Mach-Zehnder modulators can generate modulated optical signals corresponding to desired modulation schemes and thus have been put into practical use widely.

FIG. 1illustrates an example of a Mach-Zehnder modulator. In this example, the Mach-Zehnder modulator is operated as an IQ modulator. Thus, the Mach-Zehnder modulator includes an I-arm optical path and a Q-arm optical path. A Mach-Zehnder interferometer MZM_I is provided on the I-arm optical path. A Mach-Zehnder interferometer MZM_Q is provided on the Q-arm optical path. In addition, the Mach-Zehnder modulator includes a phase shifter11. The phase shifter11adjusts at least one of the phase of light propagating through the I-arm optical path or the phase of light propagating through the Q-arm optical path such that the difference between these phases becomes equal to a specified target value. In the example depicted inFIG. 1, the phase shifter11is provided on the Q-arm optical path and adjusts the phase of light propagating through the Q-arm optical path. The target value for the phase difference is, for example, π/2. Note that the following descriptions are based on the assumption that π/2 includes “π/2+2nπ (n is an arbitrary integer)”.

The Mach-Zehnder modulator generates a modulated optical signal by modulating continuous wave light generated by a light source (LD)100. In particular, the continuous wave light generated by the light source100is split and guided to the I-arm optical path and the Q-arm optical path. Then, the Mach-Zehnder interferometer MZM_I generates an optical signal I by modulating the continuous wave light with data I. The Mach-Zehnder interferometer MZM_Q generates an optical signal Q by modulating the continuous wave light with data Q. The Mach-Zehnder modulator generates a modulated optical signal by combining the optical signal I and the optical signal Q.

In this case, bias control for the Mach-Zehnder modulator is performed to generate a high-quality modulated optical signal. I-arm bias control and Q-arm bias control each configure a null point. In particular, the I-arm bias control minimizes the power of light output from the Mach-Zehnder interferometer MZM_I. The Q-arm bias control minimizes the power of light output from the Mach-Zehnder interferometer MZM_Q. Meanwhile, the phase bias control adjusts the phase difference between the I-arm optical path and the Q-arm optical path to a target value (in this example, π/2).

Bias control for a Mach-Zehnder modulator is described in, for example, International Publication Pamphlet No. WO 2017/082349, International Publication Pamphlet No. WO 2014/034047, Japanese Laid-open Patent Publication No. 2019-074612, Japanese Laid-open Patent Publication No. 2017-116746, and Japanese Laid-open Patent Publication No. 2017-188829.

A long time may be needed before a preferable state is attained when controlling the three biases indicated inFIG. 1by using the conventional technique. The operation efficiency of a communication system will be reduced if a long time is needed for bias control. For example, when a communication device is restarted due to a failure or the like in a communication system, bias control for an optical modulator may be performed. In this case, the influence of the failure could be increased if a long time is needed for the bias control.

SUMMARY

According to an aspect of the embodiments, an optical modulator includes: a Mach-Zehnder modulator; and a processor configured to control a bias of the Mach-Zehnder modulator. The Mach-Zehnder modulator includes an input optical circuit that brunches and guides input light to a first optical path and a second optical path, a first Mach-Zehnder interferometer that is formed on the first optical path, a second Mach-Zehnder interferometer that is formed on the second optical path, a phase shifter that adjusts a phase difference between the first optical path and the second optical path, and an output optical circuit that combines output light of the first Mach-Zehnder interferometer and output light of the second Mach-Zehnder interferometer to output an optical signal. The processor outputs a first bias signal for controlling an operation point of the first Mach-Zehnder interferometer, a second bias signal for controlling an operation point of the second Mach-Zehnder interferometer, and a third bias signal for controlling a phase-shift amount of the phase shifter, a low-frequency signal being superimposed on the third bias signal. The processor controls the first bias signal, the second bias signal, and the third bias signal based on a frequency component of the low-frequency signal that is included in the optical signal.

The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention.

