JP6467337B2 - Spatial phase modulation element and spatial phase modulation method - Google Patents

Description

  The present invention relates to a spatial phase modulation element used in a communication optical switch and the like, and a spatial phase modulation method.

  With the increase in communication traffic in recent years, demands for higher speed and larger capacity of optical communication are increasing, and WDM (Wavelength Division Multiplexing) transmission has been introduced as a transmission method in response thereto. In WDM transmission, it is necessary to select a path for each wavelength, and research and development of a wavelength selective switch (WSS: Wavelength Selective Switch) that performs a routing process without changing an optical signal into an electrical signal is in progress. .

  As a WSS, a spatial optical system optical switch using LCOS (Liquid Crystal on Silicon) is attracting attention (Non-patent Document 1). The LCOS is a spatial phase modulator capable of setting a phase independently for each pixel, and has a structure in which liquid crystal is sandwiched between an electrode divided for each pixel and a transparent electrode.

  FIG. 1 is a diagram showing a configuration of a spatial optical system optical switch using LCOS as a spatial phase modulator. The optical switch 100 in FIG. 1 deflects light from the input port 121 and output ports 125-1 to 12-n arranged in the x-axis direction, a lens 122 through which light from the input port 121 passes, and light from the input port 121. And a spatial phase modulator 123 that reflects the light. The light beam demultiplexed for each wavelength and emitted from the input port 121 enters the spatial phase modulator 123 via the lens 122. In the x-axis direction of the spatial phase modulator 123, a periodic sawtooth wave phase pattern 124 is set, and incident light to the spatial phase modulator 123 is modulated by the set sawtooth wave phase pattern, It is deflected and reflected. Reflected light from the spatial phase modulator 123 is connected via a lens 122 to an output port 125-i, which is one of n output port arrays. The output port to be connected is selected by changing the period of the sawtooth wave of the phase pattern 124.

G. Baxter et al., “Highly programmable Wavelength Selective Switch based on Liquid Crystal on Silicon switching elements,” OFC 2006, OTuF2, 2006.

  FIG. 2 is a diagram for explaining the phase pattern that is actually set when the phase pattern 124 of the periodic sawtooth wave having the period ω is set in the spatial phase modulator 123. The horizontal axis in FIG. 2 is the x-axis coordinate and is normalized by the pixel pitch p. The vertical axis in FIG. 2 is the phase value.

  An ideal phase waveform f (x) for deflecting light incident on the spatial phase modulator 123 in a desired direction is a sawtooth wave having a constant inclination as shown by a broken line in FIG. At this time, the phase value of f (ip) is set to the i-th pixel, and the phase pattern shown in circles in FIG. 2 is actually set. In an ideal phase waveform, the sawtooth wave is constant at a period ω, but in the actually set phase pattern, in addition to the original period ω and its harmonic components, unnecessary frequency components have a longer period than ω in particular. It can be seen that many frequency components are included. Specifically, the ideal phase waveform f (x) includes a high frequency component with respect to the spatial frequency 1 / p of the pixel. This is considered to be caused by aliasing noise generated when sampling from f (x) at the pixel pitch p.

  In such a phase pattern including an unnecessary frequency component, the diffracted light from the spatial phase modulator is generated in a direction other than the desired direction, which causes a crosstalk to be coupled to a port other than the desired output port.

  The present invention has been made in view of such a problem, and an object of the present invention is to reduce spatial crosstalk caused by the phase pattern of the spatial phase modulator deviating from the ideal pattern. To provide a phase modulator and a spatial phase modulation method.

In order to achieve such an object, a first aspect of the present invention is a spatial phase modulator used in a spatial optical system optical switch, and has a plurality of pixels arranged at equal intervals in a certain direction. A liquid crystal element that performs phase modulation with a phase pattern based on a control value set for each pixel, a controller that sets a phase pattern for each pixel, and the set phase pattern for the control value for each pixel. The controller converts and sets the spatial phase of the desired phase value sequence with respect to the desired phase value sequence set at the position of each pixel and the position between the pixels. A spatial frequency conversion unit that converts information into spatial frequency information, a filter unit that filters the spatial frequency information according to desired frequency characteristics, and the filtered spatial frequency information. A spatial phase modulator including a spatial phase converter for inverse transforming the spatial phase information, when the distance of each pixel is d, the filter unit includes a filter that cuts off frequencies above the information 1 / 2d And the desired phase value sequence is set at points of d / (2 ^ n) intervals (n is an integer of 1 or more) and 2 ^ m (m is an integer of 1 or more). To do.

In addition, a second aspect of the present invention includes a liquid crystal element having a plurality of pixels arranged at equal intervals in a certain direction and performing phase modulation by a phase pattern based on a control value set for each pixel; Spatial phase modulation in a spatial phase modulator of a spatial optical system optical switch , comprising: a controller that sets a phase pattern of each pixel; and a drive circuit that converts the set phase pattern into a control value of each pixel and sets the control value A method of converting spatial phase information of the desired phase value sequence into spatial frequency information for a desired phase value sequence set at a position of each pixel and a position between the pixels; , Filtering the spatial frequency information with a desired frequency characteristic, and inversely converting the filtered spatial frequency information into spatial phase information. Serial wherein the phase pattern is a spatial phase modulation method to be set to each pixel, if the set to the interval of the pixel d, wherein the step of filtering, 1 / 2d or more cut-off frequency information, the desired The phase value sequence is set at points of d / (2 ^ n) intervals (n is an integer of 1 or more) and 2 ^ m (m is an integer of 1 or more) .

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to suppress the crosstalk which generate | occur | produced in the high frequency area | region in the phase modulation pattern of the sawtooth wave set to a spatial phase modulator.

It is a figure which shows the structure of the spatial optical system optical switch which used LCOS as a spatial phase modulator. FIG. 3 is a diagram for explaining a phase pattern actually set when a phase pattern of a periodic sawtooth wave having a period ω is set in the spatial phase modulator of FIG. 1. It is a block diagram which shows the structure of the spatial phase modulator which concerns on one Embodiment of this invention. It is a block diagram which shows the structure of the controller of FIG. FIG. 4 is a diagram showing a state in which crosstalk between two optical switches is compared between an optical switch using a conventional spatial phase modulator and an optical switch using the spatial phase modulator of FIG. The crosstalk of the optical switch using the spatial phase modulator is shown, and (b) shows the crosstalk of the optical switch using the spatial phase modulator of FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 3 is a block diagram showing the configuration of the spatial phase modulator according to one embodiment of the present invention. The spatial phase modulator 300 in FIG. 3 is an LCOS, and includes a liquid crystal element 311, a drive circuit 312 connected to the liquid crystal element 311, and a controller 313 connected to the drive circuit 312. The liquid crystal element 311 has pixels arranged two-dimensionally, the drive circuit 312 sets a control value for each pixel so that each pixel of the liquid crystal element 311 has a specified phase value, and the controller 313 generates an appropriate phase pattern. A phase value to be set for each pixel is given to the drive circuit 312.

  The axes in which the pixels are arranged in the liquid crystal element 311 of the spatial phase modulator 300 are the x-axis and the y-axis, and the pixels are arranged at equal intervals in the x-axis direction and the y-axis direction. In the present embodiment, it is assumed that the phase pattern changes in one dimension in the x-axis direction and is uniform in the y-axis direction. Here, the phase pattern is a phase data array in which phase values set for each pixel are arrayed according to the position of the pixel. In many cases, the control value is a voltage signal, and the phase value is controlled by its amplitude and frequency.

  The spatial phase modulator 300 of FIG. 3 is used in the optical switch 100 of FIG. 1, for example, and functions as a deflecting element for incident light. In an optical switch using an LCOS as a spatial phase modulator, a periodic sawtooth phase pattern is set in the LCOS to deflect a light beam. At this time, the connection output port of the light incident on the LCOS can be switched by changing the polarization angle by changing the period ω of the phase pattern of the sawtooth wave.

  Here, when the phase pattern is set in the LCOS for deflecting the light beam, the phase pattern actually generated in the LCOS is set by being spatially discretized by the pixel and the like, as shown in FIG. As described above, a deviation from an ideal phase pattern occurs. Due to such deviation from the ideal phase pattern, the light beam is also diffracted in a direction that is not assumed as high-order light or the like, which causes optical crosstalk to ports other than the desired output port and causes deterioration of characteristics. In this embodiment, in order to suppress optical crosstalk to ports other than the desired output port, a function for correcting the phase pattern of a periodic sawtooth wave is implemented.

  FIG. 4 is a block diagram showing a configuration of the controller 313 in FIG. 4 is connected to the phase pattern generation unit 451, the spatial frequency conversion unit 452 connected to the phase pattern generation unit 451, the filter unit 453 connected to the spatial frequency conversion unit 452, and the filter unit 453. And a space area conversion unit 454.

  When the phase pattern generation unit 451 receives a phase pattern generation instruction from the outside, the controller 313 generates spatial phase information (spatial phase domain function) that is a phase value sequence of discrete data from the ideal phase waveform f (x). To do. Discrete data generates not only the pixel center position but also the phase waveform between the pixel center positions.

  Next, the spatial frequency conversion unit 452 converts the spatial phase information of the phase value sequence into the spatial frequency information for each pixel position and a desired phase value sequence (phase waveform) set between the pixel center positions. . Specifically, discrete data (spatial phase domain function) generated by the phase pattern generation unit 451 is converted into a spatial frequency domain function F (γ) (γ is a spatial frequency). For the conversion to the spatial frequency domain function F (γ), a method of applying digital processing such as Fast Fourier Transform (FFT) is used. Therefore, the phase pattern generation unit 451 may generate data with f (x) as f (p / s) (s is an integer). Furthermore, the phase value sequence is set at points of d / (2 ^ n) interval (n is an integer of 1 or more) and 2 ^ m (m is an integer of 1 or more), that is, s is a multiplier of 2. By calculating as 4, 8,... (2 ^ n (n is an integer of 1 or more)), the accuracy of FFT can be improved.

  Next, the filter unit 453 filters the spatial frequency information (spatial frequency domain function F (γ)) of the phase value sequence with a desired frequency characteristic. Specifically, the high frequency is cut off so that the spatial frequency domain function F (γ) converted by the spatial frequency conversion unit 452 does not have a component of the spatial frequency 1 / 2p or higher so as not to generate aliasing noise. Perform filtering processing.

  Finally, in the spatial domain conversion unit 454, the spatial frequency information (spatial frequency domain function F (γ)) filtered by the filter unit 453 is inversely converted again into spatial phase information (spatial phase domain function), The set phase of the pixel is determined. For inverse transformation, Fourier inverse transformation is usually used. The set phase value is transmitted to the drive circuit 312.

  FIG. 5 is a diagram showing a state in which crosstalk of two optical switches is compared between an optical switch using the conventional spatial phase modulator 123 and an optical switch using the spatial phase modulator 300 of FIG. . Here, FIG. 5A shows the crosstalk of the optical switch using the conventional spatial phase modulator 123, and FIG. 5B shows the crosstalk of the optical switch using the spatial phase modulator 300 of FIG. Is shown. As the ideal phase waveform of the periodic sawtooth wave phase pattern, the sawtooth wave is used, the phase slope (frequency) of the sawtooth wave is on the horizontal axis, and the vertical axis is the output port number of the optical switch. The coupling power of is displayed. It can be seen that the crosstalk generated in the region where the phase slope of the sawtooth wave is high, that is, the frequency is high, in the conventional method is suppressed in the present invention.

  In the present embodiment, frequency domain conversion and filtering are performed on the ideal phase waveform. However, the same applies to the phase waveform in which the phase is constant with respect to x in each pixel, as indicated by the solid line in FIG. Even when this processing is performed, aliasing noise can be suppressed, and the same effect as in the present embodiment can be expected.

  In the present embodiment, the phase pattern changes in one dimension in the x-axis direction and is uniform in the y-axis direction. However, even in the phase pattern that changes in two dimensions in xy, x and y respectively. By performing the filtering process shown in the present embodiment in the direction of, unnecessary light can be suppressed and crosstalk can be reduced.

DESCRIPTION OF SYMBOLS 100 Optical switch 121 Input port 122 Lens 123, 300 Spatial phase modulator 124 Periodic phase pattern 125-1 to 125-n Output port 311 Liquid crystal element 312 Drive circuit 313 Controller 451 Phase pattern generation part 452 Spatial frequency conversion part 453 Filter part 454 Spatial domain converter

Claims (2)

A spatial phase modulator used in a spatial optical system optical switch,
A liquid crystal element having a plurality of pixels arranged at equal intervals in a certain direction and performing phase modulation by a phase pattern based on a control value set for each pixel;
A controller for setting a phase pattern of each pixel;
A drive circuit that converts and sets the set phase pattern into a control value for each pixel;
The controller is
A spatial frequency converter that converts spatial phase information of the desired phase value sequence into spatial frequency information for a desired phase value sequence set at the position of each pixel and a position between the pixels;
A filter unit for filtering the spatial frequency information by a desired frequency characteristic;
The spatial frequency filtered information is including spatial phase modulator and a spatial domain transforming section for inversely converting the spatial phase information,
When the interval between the pixels is d, the filter unit is a filter that cuts off frequency information of 1 / 2d or more,
The desired phase value sequence is set at points of d / (2 ^ n) interval (n is an integer of 1 or more) and 2 ^ m (m is an integer of 1 or more). Spatial phase modulator. A liquid crystal element having a plurality of pixels arranged at equal intervals in a certain direction and performing phase modulation by a phase pattern based on a control value set for each pixel;
A controller for setting a phase pattern of each pixel;
A spatial phase modulation method in a spatial phase modulator of a spatial optical system optical switch including a drive circuit that converts and sets the set phase pattern into a control value of each pixel,
Converting the spatial phase information of the desired phase value sequence into spatial frequency information for the desired phase value sequence set at the position of each pixel and the position between the pixels;
Filtering the spatial frequency information by a desired frequency characteristic;
The spatial frequency modulation method of setting the phase pattern to each pixel by inversely transforming the filtered spatial frequency information into spatial phase information ,
When the interval between the pixels is d, the filtering step cuts off frequency information of 1 / 2d or more,
The desired phase value sequence is set at points of d / (2 ^ n) interval (n is an integer of 1 or more) and 2 ^ m (m is an integer of 1 or more). Spatial phase modulation method.