Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/08—Time-division multiplex systems
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
  • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
  • G02B6/2861—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using fibre optic delay lines and optical elements associated with them, e.g. for use in signal processing, e.g. filtering
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/005—Optical Code Multiplex

Definitions

• the present invention relates to an optical circuit, and more particularly to an optical functional circuit which performs a desired operation on an inputted optical signal in the optical domain.
• the optical functional circuit represents a notion which encompasses an optical code division multiplexing circuit, an optical code division demultiplexing circuit, an optical time division multiplexing circuit, an optical time division demultiplexing circuit, an optical phase control circuit for arrayed antennas, an optical digital-to-analog conversion circuit, and an optical routing circuit.
• FIG. 13 is a block diagram showing the structure of a conventional optical code division multiplexing circuit 900.
• the conventional optical code division multiplexing circuit 900 shown in FIG. 13 is disclosed in, for example, Japanese Patent No. 3,154,396.
• the conventional optical code division multiplexing circuit 900 includes an optical splitting section 901, first through N ' th optical fibers 902-1 through 902-N, and an optical combining section 903.
• the optical splitting section 901 splits an input optical signal into N optical signals, and outputs them.
• the first through N'th optical fibers 902-1 through 902-N are of different optical path lengths . Accordingly, the first through N ' th optical fibers 902-1 through 902-N cause their respective different amounts of delay.
• the optical code division multiplexing circuit 900 is assigned with a predetermined code.
• the number N of branches in the optical splitting section 901 and the amounts of delay caused in the first through N'th optical fibers 902-1 through 902-N are determined in accordance with the assigned predetermined code. In the example shown in FIG. 13, it is assumed that an M-bit sequence "1101...11" is assigned as the predetermined code. In this case, the number N_ of branches agrees with the total number of
• the amount of delay in the first optical fiber 902-1 is ⁇
• the amount of delay in the second optical fiber 902-2 is 2 ⁇ ⁇
• the amount of delay in the third optical fiber 902-3 is 4 ⁇ ⁇
• the amount of delay in the N-l'th optical fiber 902-N-l is (M-l) ⁇ ⁇
• the amount of delay in the N'th optical fiber 902-N is Mx ⁇ .
• the first through N'th optical fibers 902-1 through 902-N apply their respective predetermined amounts of delay to optical signals outputted from the optical splitting section 901, and output first through N'th optical delayed signal.
• the structure of an 'th optical fiber 902-m conforms to that of the first optical fiber 902-1.
• the optical combining section 903 combines the N optical delayed signals outputted from the first through N'th optical fibers 902-1 through 902-N, and outputs a resultant signal. In this manner, a spread-spectrum optical signal is outputted from the optical combining section 903.
• the number N of optical branches and lengths of optical fibers i.e. , amounts of delay
• a conventional optical functional circuit such as an optical code division multiplexing circuit, which exploits propagation delay in optical fibers, uses optical fibers of different lengths to adjust the amounts of delay.
• optical code division multiplexing circuit which exploits propagation delay in optical fibers, uses optical fibers of different lengths to adjust the amounts of delay.
• lengths of optical fibers to be included vary for each predetermined code, therefore it is required to change the layout for arranging the optical fibers for each product, leading to an increase in production cost.
• the amounts of delay are determined in accordance with the lengths of the optical fibers. Accordingly, in the conventional optical functional circuit, it is substantially impossible to later adjust the amounts of delay with high precision, leading to a low productivity rate (i.e., a rate of production output to raw materials used) .
• an object of the present invention is to provide a small-sized optical functional circuit which eliminates' the need to change the layout of waveguides on a product-by product basis and ensures a high productivity rate.
• the present invention has the following aspects.
• a first aspect of the present invention is directed to an optical functional circuit which performs a delay process on N optical signals, where N is an integer equal to or more than 1, to obtain N optical delayed signals, the circuit comprising N delay waveguides each having an amount of delay assigned thereto, wherein at least one of the N delaywaveguides includes at least one photonic crystal, and wherein the photonic crystal is structured such that a refractive index thereof varies periodically in accordance with an assigned amount of delay.
• the photonic crystal having a periodically varying refractive index functions as a delay waveguide due to an optical confinement effect.
• the delaywaveguide is formedbythe photonic crystal, whereby it is possible to provide an optical functional circuit whose circuit scale is smaller than in the conventional art. Also, it is possible to provide an optical functional circuit the layout of which can be readily designed. Also, the optical device length is consistent amongdelaywaveguides includingthephotonic crystal, whereby it is possible to provide an optical functional circuit which eliminates the need to change the layout of delay waveguides for each assigned code. Moreover, it is possible to later adjust the delay waveguides including the photonic crystal, leading to an increase of the productivity rate.
• the photonic crystal includes a plurality of first holes arranged substantially in a periodical manner and a second hole provided so as to disturb the periodicity of the plurality of first holes.
• a portion in the vicinity of the second hole acts as a microresonator, and the photonic crystal is able to cause a delay on an incoming optical signal.
• the plurality of first holes consist of a first group of holes arranged in a straight line at regular intervals and a second group of holes opposed to the first group with respect to a shift region andarranged ina straight line at regular intervals, and the second hole is located within the shift region.
• the shift region acts as a microresonator, and the photonic crystal is able to cause a delay on an incoming optical signal .
• two or more of the N delay waveguides include one or more photonic crystals
• the one or more photonic crystals are each a basic unit for amount of delay
• at least one of the two or more of the N delay waveguides includes photonic crystals arranged in series.
• the delay waveguides are optical waveguides of the same optical device length, and each of the optical waveguides includes a photonic crystal which is a basic unit for amount of delay assigned thereto.
• the photonic crystal includes a plurality of first holes arranged at regular intervals and a plurality of second holes arranged at regular intervals, and the first and second holes are arranged so as to alternate with each other.
• the refractive index of the photonic crystal varies periodically.
• the photonic crystal includes crystals of different refractive indices.
• the refractive index of the photonic crystal varies periodically.
• the photonic crystal includes layers of different refractive indices.
• the refractive index of the photonic crystal varies periodically.
• the optical functional circuit further comprises: an optical splitting section for splitting an input optical signal into N optical signals, and inputting the N optical signals to the delay waveguides; and an optical combining section for combining N optical delayed signals outputted from the delay waveguides, wherein the amounts of delay differ from each other in accordance with a code assigned for spread spectrum.
• an optical code division multiplexing circuit capable of spread spectrum of an input optical signal in accordance with an assigned code.
• the optical functional circuit further comprises: an optical splitting section for splitting an input optical signal subjectedto spread spectrum into N optical signals, and inputting the N optical signals to the delay waveguides; and an optical combining section for combining N optical delayed signals outputted from the delay waveguides, and the amounts of delay differ from each other in accordance with a code assigned for inverse spread spectrum.
• an optical code division demultiplexing circuit capable of inverse spread spectrum of an input optical signal subjected to spread spectrum.
• the optical functional circuit further comprises : a wavelength separation section for separating an input optical signal having a plurality of wavelength components into different wavelengths, and inputting the different wavelengths as the N optical signals to the delay waveguides; and an optical combining section for combiningN optical delayed signals outputted from the delay waveguides, wherein the amounts of delay differ from each other in accordance with a code assigned for spread spectrum.
• a wavelength separation section for separating an input optical signal having a plurality of wavelength components into different wavelengths, and inputting the different wavelengths as the N optical signals to the delay waveguides
• an optical combining section for combiningN optical delayed signals outputted from the delay waveguides, wherein the amounts of delay differ from each other in accordance with a code assigned for spread spectrum.
• the optical functional circuit further comprises : a wavelength separation section for separating an input optical signal subjected to spread spectrum into different wavelengths, and inputting the different wavelengths as the N optical signals to the delay waveguides; and an optical combining section for combining N optical delayed signals outputted from the delay waveguides, wherein the amounts of delay differ from each other in accordance with 'a code assigned for inverse spread spectrum.
• a wavelength separation section for separating an input optical signal subjected to spread spectrum into different wavelengths, and inputting the different wavelengths as the N optical signals to the delay waveguides
• an optical combining section for combining N optical delayed signals outputted from the delay waveguides, wherein the amounts of delay differ from each other in accordance with 'a code assigned for inverse spread spectrum.
• the optical functional circuit further comprises: an optical splitting section for splitting an input optical signal into N optical signals, and inputting the N optical signals to the delay waveguides; and an optical combining section for combining N optical delayed signals outputted from the delay waveguides, wherein the delay waveguides each transmit therethrough only an optical signal whose wavelength is different from those of optical signals transmitted through other delay wavelengths, and wherein the amounts of delay differ from each
• the optical functional circuit further comprises: an optical splitting section for splitting an input optical signal subjected to spread spectrum into N optical signals, and inputting the N optical signals to the delay waveguides; and an optical combining section for combining N optical delayed signals outputted from the delay waveguides, wherein the delay waveguides each transmit therethrough only an optical signal whose wavelength is different from those of optical signals transmitted through other delay wavelengths, and wherein the amounts of delay differ fromeach other in accordance with a code assigned for inverse spread spectrum.
• the optical functional circuit further comprises: a clock generation section for- generating an optical clock signal; a clock splitting section for splitting the optical clock signal generated by the clock generation section into N optical clock signals; N clock combining sections for combining N optical data signals with the N optical clock signals obtained by the clock splitting section, and inputting N optical signals to their corresponding delay waveguides; and an optical combining section for combining N optical delayed signals outputted from the delay waveguides, wherein the amounts of delay are different integral multiples of a minimum amount of delay.
• the optical functional circuit further comprises: a clock generation section for generating an optical clock signal; a clock splitting section for splitting the optical clock signal generated by the clock generation section into N optical clock signals; an optical splitting section for splitting an input optical signal subjected to optical time division multiplexing into N optical signals, and inputting the N optical signals to the delay waveguides; and N clock combining sections for combining N optical delayed signals outputted from the delay waveguides with the N optical clock signals outputted from the clock splitting section, wherein the amounts of delay are different integral multiples of a minimum amount of delay.
• the optical functional circuit further comprises:- a radio-to-pptical conversion section for converting an incoming radio signal into a radio-over-optical signal; an optical splitting section for splitting the radio-over-optical signal outputted fromthe radio-to-optical conversion section into N optical signals, and inputting the N optical signals to the delay waveguides; N optical-to-radio conversion sections for converting N optical delayed signals outputted from the delay waveguides into N radio signals; and N array antennas each being connected to a corresponding one of the N optical-to-radio conversion sections and emitting a radio signal outputted from the corresponding optical-to-radio conversion section toward a direction corresponding to a phase of the radio signal.
• the optical -functional circuit further comprises: an optical splitting section for splitting an incoming optical digital-waveform signal into N optical signals, and inputting the N optical signals to the delaywaveguides; N weighting sections provided in association with the N delay waveguides and adjusting amplitudes of N optical delayed signals outputted from the N delay waveguides such that N optical delayed signals having a predetermined amplitude ratio are outputted from the N weighting sections; and an optical combining section for combining the N optical delayed signals whose amplitudes have been adjusted and which have' been outputted from the weighting sections, wherein the amounts of delay are different integral multiples of a minimum amount of delay.
• an optical digital-analog conversion circuit capable of converting an input optical digital-waveform signal into an optical analog-waveform signal.
• information contained in an unconverted digital-waveform is represented by the magnitude of analog amplitude at a time gate where the bit rate is lower than that compared to the digital-waveform. Accordingly, a device having received an optical analog-waveform signal is able to perform processing at a speed lower than that on the transmitter side.
• N is 1, the circuit further comprises: an optical splitting section for splitting an input optical signal containing label information into two optical signals, and inputting one of the two optical signals to a delay waveguide; a label recognition section for recognizing the label information based on another of the two optical signals obtained by the optical splitting section; and an optical path switching section having a plurality of output ports and switching between the output ports based on a result of recognition by the label recognition section to select an output port to which an optical delayed signal outputted from the delay waveguide is outputted, and the delay waveguide has assigned thereto an amount of delay required for label recognition by the label recognition section.
• an optical routing circuit capable of switching paths for an input optical signal based on the label information.
• each delay waveguide includes a waveguide layer composed of silicon and a clad layer composed of silicon dioxide (Si0 2 ) .
• each delay waveguide may include an optical switching section for switching whether to output or not.
• the optical functional circuit may further comprise a delay offset section for causing a delay on an input optical signal, and the N optical signals may be previously delayed by. the delay offset section. In this case, it is possible to reduce the number of photonic crystals provided in a delay waveguide.
• a second aspect of the present invention is directed to a delay waveguide for causing a delay on an incoming optical signal, the waveguide comprising at least one photonic crystal which is structured suchthat a refractive indexthereofvaries periodically in accordance with a desired amount of delay.
• the photonic crystal having a periodically varying refractive index functions as a delay waveguide due to an optical confinement effect.
• the delay waveguide is small and therefore can be applied to various optical functional circuits. Also, later adjustments are possible, and therefore the productivity rate of the delay waveguide can be improved.
• the photonic crystal includes : a plurality of first holes arranged at regular intervals; and a second hole provided so as to disturb a periodicity of the plurality of first holes.
• a portion in the vicinity of the second hole acts as a microresonator, and the photonic crystal is able to cause a delay on an incoming optical signal.
• the plurality of first holes consist of a first group of holes arranged in a straight line at regular intervals and a second group of holes opposed to the first group with respect to a shift region andarranged in a straight line at regular intervals, and the second hole is located within the shift region.
• the shift region acts as a microresonator, and the photonic crystal is able to cause a delay on an incoming optical signal.
• the delay waveguide include two or more photonic crystals, the two or more photonic crystals are each a basic unit for amount of delay, and the two or more photonic crystals are arranged in series.
• it is possible to cause a desired amount of delay by merely forming a predetermined number of photonic crystals, which are basic units, in accordance with the amount of delay, making it possible to readily produce the delay waveguide.
• the photonic crystal includes: a plurality of first holes arranged substantially in a periodical manner; and a second hole provided so as to disturb the periodicity of the pluralityof first holes, andthe first and secondholes are arranged so as to alternate with each other.
• the refractive index of the photonic crystal varies periodically.
• the photonic crystal includes crystals of different refractive indices.
• the refractive index of the photonic crystal varies periodically.
• the photonic crystal includes layers of different refractive indices.
• the refractive index of the photonic crystal varies periodically.
• an optical functional circuit of the present invention is structured such that delay waveguides include photonic crystals, and therefore it is possible to obtain different amounts of delay while keeping the same optical device length, leadingto a small circuit scale. Further, even if delaywaveguides have their respective different amounts of delay assigned thereto, it is not necessary to change the layout of the waveguides. Furthermore, later processing of an adjustment hole makes it possible to later adjust the loss or the like after the production of the circuit.
• FIG. 1 is a block diagram showing a structure of an optical code division multiplexing circuit 1 according to the first embodiment of the present invention
• FIG. 2 is a block diagram showing a structure of an optical code division demultiplexing circuit 2 on the receiver side which is intended for inverse spread spectrum of an input optical signal which has been subjected to spread spectrum and transmitted from the optical code division multiplexing circuit 1 shown in FIG. 1
• FIG. 3 is a schematic view showing an exemplary structure of a photonic crystal 121
• FIG. 4 is a schematic view showing another exemplary arrangement pattern of holes
• FIG. 5 is a block diagram showing a structure of an optical code division multiplexing circuit 3 for spread spectrum of an input optical signal of a wide wavelength band
• FIG. 6 is a block diagram showing a structure of an optical code division demultiplexing circuit 4 for inverse spread spectrum of an input optical signal subjected to spread spectrum
• FIG. 7 is a block diagram showing a structure of an optical time division multiplexing circuit 5 according to a third embodiment of the present invention
• FIG. 8 is a block diagram showing a structure of an optical time division demultiplexing circuit 6 according to the third embodiment of the present invention
• FIG. 9 is a block diagram showing a structure of an optical phase control circuit 7 for arrayed antennas according to a fourth embodiment of the present invention
• FIG.10 is a block diagram showing a structure of an optical digital-to-analog conversion circuit 8 according to a fifth embodiment of the present invention
• FIG. 11 is a diagram for explaining exemplary signal flows in the optical digital-to-analog conversion circuit 8;
• FIG.12 is a block diagram showing a structure of an optical routing circuit 9 according to a sixth embodiment of the present invention; and
• FIG. 13 is a block diagram showing the structure of a conventional optical code division multiplexing circuit 900.
• Each optical functional circuit described in the following embodiments includes N delay waveguides (where N is an integer equal to or more than 1) .
• the delay waveguides perform a process of causing a delay on N optical signals to obtain N optical delayed signals.
• the N delay waveguides each are previously assigned with an amount of delay.
• Each delay waveguide includes at least one photonic crystal. The photonic crystal is structured such that the refractive index thereof varies periodically in accordance with the amount of delay assigned to the delay waveguide. Note that in FIGs.
• FIG. 1 is a block diagram showing a structure of an optical code division multiplexing circuit 1 according to the first embodiment of the present invention.
• FIG. 1 flows of signals in the case of spread spectrum of an optical signal are shown. That is, the optical code division multiplexing circuit 1 is a circuit on the transmitter side.
• FIG. 1 flows of signals in the case of spread spectrum of an optical signal are shown. That is, the optical code division multiplexing circuit 1 is a circuit on the transmitter side.
• the optical code division multiplexing circuit 1 includes an optical splitting section 11, first through N'th delay waveguides 12-1 through 12-N, and an optical combining section 13.
• a delay waveguide 12-m (where m is an integer from 1 to N) is an optical waveguide capable of causing a delay.
• the optical waveguide includes one or more photonic crystals 121 which are arranged in series.
• the optical waveguide is typically composed of a waveguide layer, which transmits light therethrough, and a clad layer for supporting it.
• the waveguide layer is composed of silicon.
• the clad layer is composed of silicon dioxide (Si0 2 ) .
• the optical code division multiplexing circuit 1 is previously assigned with a code for spread spectrum.
• the number N of branches in the optical splitting section 11 is determined in accordance with the assignedpredetermined code. Also, amounts of delay for first through N'th delay waveguides are determined in accordance with the assigned predetermined code. The amounts of delay are different from each other. For example, if the assigned predetermined code is "101", the number N of branches is 2. The amount of delay in the first delay waveguide 12-1 is ⁇ , and the amount of delay in the second delay waveguide 12-2 is 3 ⁇ . That is, the number N of branches agrees with the total number of "Is" in the assigned predetermined code. The amount of delay in each delay waveguide corresponds to a bit position in the predetermined code where "1" is shown. In the example shown in FIG.
• the rule of assigning amounts of delay is not limited to the above so long as the assignment rule allows optical code division multiplexing. Note that optical pulses are required to be inputted to the optical splitting section 11 at intervals greater than the maximum amount of delay in the first through N'th delay waveguides 12-1 through 12-N.
• the pulses are required to be inputted at intervals equal to or more than M ⁇ .
• the optical splitting section 11 splits an input optical signal into N optical signals, and outputs them.
• the first through N'th optical waveguides 12-1 through 12-N apply their respective predetermined amounts of delay to the incoming N optical signals, and output N optical delayed signals.
• the structure of an N'th delay waveguide 12-n conforms to that of the N'th delay waveguide 12-N.
• the optical combining section -13 combines the N optical delayed signals outputted from output terminals of the first through N'th delay waveguides 12-1 through 12-N, and outputs a resultant signal.
• the optical code divisionmultiplexing circuit 1 one pulse having entered the optical splitting section 11 is divided into a string of N pulses to obtain N optical delayed signals having been delayed based on a predetermined code, and the optical combining section 13 generates an optical signal (output signal) subjected to spread spectrum on the time axis. Accordingly, a code contained in an optical signal subjected to spread spectrum can be freely changed by adjusting the number N of optical branches and amounts of delay in the N delay waveguides .
• FIG. 1 is a block diagram showing a structure of an optical code division demultiplexing circuit 2 on the receiver side which is intended for inverse spread spectrum of an input optical signal which has been subjected to spread spectrum and transmitted from the optical code division multiplexing circuit 1 shown in FIG. 1.
• the optical code division demultiplexing circuit 2 is intended for inverse spread spectrum of an input optical signal which has been subjected to spread spectrum and transmitted from the optical code division multiplexing circuit 1 shown in FIG. 1.
• the optical code division demultiplexing circuit 2 includes an optical splitting section 21, first through N'th delay waveguides 22-1 through 22-N, and an optical combining section 23.
• An m'th delay waveguide 22-m (where m is an integer from 1 to N) includes photonic crystals 121 which are arranged in series.
• the optical code division demultiplexing circuit 2 is previously assigned with a predetermined code for inverse spread spectrum.
• the first through N'th delay waveguides 22-1 through 22-N in the optical code division demultiplexing circuit 2 are assigned with amounts of delay complementary to the amounts of delay assigned to the first throughN ' th delaywaveguides 12-1 through 12-N in the optical code division multiplexing circuit 1 on the transmitter side.
• the amounts of delay assigned to the first through N'th delay waveguides 12-1 through 12-N of the optical code division multiplexing circuit 1 on the transmitter side are assumed to be a (1) ⁇ , a(2) ⁇ ⁇ , a(3) ⁇ ⁇ , ... , a(N-l) ⁇ , anda(N) ⁇ ⁇
• the amounts of delay assigned to the first through N'th delay waveguides 22-1 through 22-N of the optical code division demultiplexing circuit 2 on the receiver side are assumed to be b(l) ⁇ ⁇ ,b(2) ⁇ ⁇ ,b(3) ⁇ ⁇ , ... , b (N-l) x ⁇ , and b (N) x ⁇ .
• T is a constant, (time) .
• the optical splitting section 21 splits an input optical signal subjected to spread spectrum into N optical signals, and outputs them.
• the first throughN ' th delaywaveguides 22-1 through 22-N apply their respective predetermined amounts of delay to the N optical signals, and output N optical delayed signals.
• the optical combining section 23 combines the N optical delayed signals outputted from output terminals of the first through N'th delay waveguides 22-1 through 22-N, and outputs a resultant signal.
• the amounts of delay in the first through N'th delay waveguides 22-1 through 22-N on the receiver side are complementary to the amounts of delay in the first through N'th delay waveguides 12-1 through 12-N on the transmitter side. Therefore, in the optical combining section 23, the optical signals obtained by the optical splitting section 21 reach a peak (hereinafter, referred to as a "correlation peak") at time corresponding to the above constant T.
• a peak hereinafter, referred to as a "correlation peak
• the optical code division demultiplexing circuit 2 shown in FIG.2 delay waveguides assigned with complementary amounts of delay are used to obtain the correlation peak.
• data reconstruction can be achieved by inverse spread spectrum.
• FIG. 3 is a schematic view showing an exemplary structure of the photonic crystal 121.
• the photonic crystal 121 has holes drilled by a short pulse laser.
• the photonic crystal 121 in an optical waveguide includes a plurality of circular holes (first holes) 122 and an adjustment hole (second hole) 123.
• the circular holes 122 are drilled in the optical waveguide using an ultra-short pulse laser such as a femtosecond laser.
• the circular holes 122 are classified into a first hole group 122a consisting of holes arranged in a straight line at regular intervals, and a second hole group 122b opposed to the first hole group 122a with respect to a shift region and consisting of holes arranged ⁇ in a straight line at regular intervals.
• the circular holes 122 included in each of the first and second hole groups 122a and 122b are arranged along a one-dimensional direction so as to correspond to a period (hereinafter, referred to as a "lattice constant") equal to merely a fraction of a wavelength (e.g., a 1550 nanometer (nm) band) of an optical signal desired to transmit .
• the circular holes 122 each have a diameter which is equal to merely a fraction of a wavelength (e.g., a 1550 nanometer (nm) band) of an optical signal desired to transmit.
• the adjustment hole 123 is drilled in the optical waveguide using an ultra-short pulse laser such as a femtosecond laser, so as to disturb the periodicity of the circular holes 122.
• the circular holes 122 each have a diameter equal to merely a fraction of a wavelength (e.g., a 1550 nm band) of an optical signal desired to transmit, and they are arranged at regular intervals of merely a fraction of thewavelength of an optical signal desiredto transmit .
• the adjustment hole 123 has a depth different from those of the circular holes 122, or a diameter different from those of the circular holes 122.
• the adjustment hole 123 is provided in the shift region.
• the photonic crystal 121 as described above is structured such that the refractive index thereof varies periodically, and functions as a delay element which utilizes an optical filter effect .
• a Q-factor representing the characteristic of an optical filter is defined by peak wavelength ⁇ c, and half width ⁇ of the transmittance characteristic.
• the amount of delay D of the optical filter has a proportionality as shown in the following expression (2) using a Q-factor, light velocity c, and circular constant ⁇ .
• the diameter of the circular hole 122 is approximately 200 nm
• a shift region of about 630 nm in length is provided to disturb the periodicity of the circular hole 122
• the adjustment hole 123 is provided in the shift region.
• the shift region where the periodicity is disturbed becomes an opaque region (bandgap region) and a transparent region having a sharp wavelength transmission characteristic appears in the vicinity of the adjustment hole 123 within the opaque region.
• the opaque region acts as a microresonator .
• the photonic crystal 121 is formed in this manner, it is possible to obtain a photonic crystal whose half-width ⁇ is about 2 nm.
• the photonic crystal 121 acts as an optical filter whose Q-factor is about 700.
• the amount of delay of the photonic crystal 121 is proportional to about 1 picosecond (ps) .
• the photonic crystal 121 causes a microresonance phenomenon, and therefore functions as an optical delay element.
• delays caused by the microresonance phenomenon are schematically shown.
• One photonic crystal 121 is a basic unit for optical delay.
• the first through N'th delay waveguides 12-1 through 12-N or 22-1 through 22-N each are assigned with a desired amount of delay.
• a photonic crystal adding an amount of delay of 1 ps is used as a basic unit, if it is desired to assign an amount of delay of m ps to an m'th delay waveguide 12-m, it is required to serially arrange m photonic crystals 121 on the optical waveguide.
• a plurality of optical waveguides each including one photonic crystal 121 may be connected in series to serially arrange a plurality of photonic crystals 121, or a plurality of photonic crystals 121 may be formed in one optical waveguide using a short pulse laser.
• delaywaveguides may be formed by optical waveguides of the same optical device length, and each delay waveguide may include a photonic crystal 121 which corresponds to a basic unit of the amount of delay assigned to the delay waveguide.
• the first through N'th delay waveguides 12-1 through 12-N or 22-1 through 22-N may be formed so as to include photonic crystals in which the periodicity or number of circular holes 122 or the depth or diameter of the adjustment hole are modified in accordance with the amount of optical delay. Accordingly, the first through N'th delay waveguides 12-1 through 12-N or 22-1 through 22-N are not limited to the above examples so long as the amount of delay is determined by the photonic crystals.
• a delay circuit is configured by utilizing propagation delay in optical fibers
• ⁇ m micrometers
• the photonic crystals 121 as described in the first embodiment it is possible to use delay waveguides having a length of several tens of micrometers, making it possible to provide a small-sized optical functional circuit.
• a desired amount of delay time is obtained by adjusting the number of photonic crystals to be formed, the periodicity of the circular holes 122, or the depth or diameter of the adjustment hole 123. Accordingly, it is possible to use delay waveguides of the same optical device length. Therefore, unlike the conventional art in which optical fibers of different lengths are laid out, it is possible to provide an optical functional circuit the layout of which can be readily designed.
• the adjustment hole 123 of the photonic crystal 121 is later processed to adjust the amount of delay or loss, making it possible to provide an optical functional circuit which can be adjusted after production. As a result, the productivity rate is improved.
• the following methods are conceivable as methods for later processing the photonic crystals.
• the adjustment hole 123 is further irradiated with a short pulse laser beam. The first method allows the amount of delay or the transmittance to be later adjusted.
• a reflection hole is provided away from the adjustment hole 123.
• the second method allows the transmittance to be later adjusted.
• all delay waveguides include at least one photonic crystal.
• the optical device length of the delay waveguide ' having the photonic crystal does not change, it is possible to provide an optical functional circuit in which the layout of the delay waveguides are not required to be changed for each assigned code.
• the productivity rate can be improved. Note that the properties of the photonic crystal vary in accordance with a change of temperature, and therefore the first through N'th delay waveguides 12-1 through 12-N or 22-1 through
• the circular holes 122 and the adjustment hole 123 are holes in the shapes of circles which are arranged in a straight line in a one-dimensional direction.
• the circular holes 122 and the adjustment hole 123 may be arranged in a two-dimensional manner so long as a similar delay effect can be achieved.
• the shape of a hole may be optional, and can be an ellipse or a triangle. Also, holes of different shapes can be combined.
• the photonic crystal is not limited to the structure shown in FIG.3 so long as it includes a plurality of first holes arranged substantially in a periodical manner and a second hole provided so as to disturb the periodicity of the plurality of first holes.
• the refractive index varies periodically, making itpossible to cause a delay on an input optical signal due to the optical confinement effect.
• FIG. 4 is a schematic view showing another exemplary arrangement pattern of holes. As shown in FIG. 4, the photonic crystal includes a plurality of first holes 124 arranged at regular intervals and a plurality of second holes 125 arranged at regular intervals. The first holes 124 and the second holes 125 alternate with each other.
• the refractive index varies periodically, making it possible to cause a delay on an input optical signal due to the optical confinement effect. Further, even in the case of a photonic crystal which includes crystals of different refractive indices and arranged at regular intervals, it is possible to cause a delay on an input optical signal.
• the refractive index is caused to vary periodically by irradiating the photonic crystal with laser at regular intervals to such an extent as not to bore a hole therethrough.
• the photonic crystal including crystals of different refractive indices means a photonic crystal crystallized from one substancewhose crystal structurevaries periodicallyor aphotonic crystal crystallized from a plurality of substances in which crystals of different substances are arranged periodically.
• the layers of different refractive indices may be a mulitlayer mirror or the like.
• the photonic crystal having layers of different refractive indices at regular intervals means a photonic crystal composed of layers of different structures or thicknesses . Each layer may or may not be composed of a substance of the same kind.
• the above-described hole processing technique using a short pulse laser is disclosed in Japanese Laid-Open Patent Publication No.2002-18585 or Ming Li et al .
• holes of the photonic crystal are not limited to those made by direct laser writing. Holes can be formed in the photonic crystal not only by direct laser writing but also by processing by electronic beam exposure or UV lithography processing, for example. However, in the case of electronic beam exposure or UV lithography, there are difficulties inprocessing inthe atmosphere or performing later adjustments.
• the optical splitting section 11 or 21 may include a delay offset section for applying a portion of- a delay that is assigned to the first through N'th delay waveguides 12-1 through 12-N or 22-1 through 22-N. This makes it possible to reduce the number of photonic crystals included in the first through N'th delay waveguides 12-1 through 12-N or 22-1 through 22-N .
• the delay offset section may use the above-described photonic crystal to apply the delay. Also, the delay offset section is not required to be included in either the optical splitting section 11 or 21 so long as it is disposed so as to cause a delay on an input optical signal before the optical signal enters each delay waveguide. By providing the delay offset section, N optical signals inputted to the delay waveguides are previously delayed.
• FIG. 5 is a block diagram showing a structure of an optical code division multiplexing circuit 3 for spread spectrum of an input optical signal of a wide wavelength band.
• FIG. 6 is a block diagram showing a structure of an optical code division demultiplexing circuit 4 for inverse spread spectrum of an input optical signal subjected to spread spectrum.
• the structures of the optical code division multiplexing circuit 3 shown in FIG. 5 and the optical code division demultiplexing circuit 4 shown in FIG. 6 are respectively similar to those of the optical code division multiplexing circuit 1 shown in FIG. 1 and the optical code division demultiplexing circuit 2 shown in FIG. 2, except that the optical splitting sections 11 and 21 shown in FIGs .1 and 2 are replaced with wavelength separation sections 31 and 41, respectively.
• elements similar to those shown in FIGs. 1 and 2 are denoted by the same reference numerals .
• Amounts of delay assigned to the first through N ' th delay waveguides 12-1 through 12-N and the first through N ' th delay waveguides 22-1 through 22-N are the same as those in the examples illustrated in FIGs. 1 and 2.
• the wavelength separation section 31 separates an input optical signal having a plurality of wavelength components into the different wavelengths, and outputs them.
• the first throughN'th delaywaveguides 32-1 through 32-N delay optical signals inputted thereto by an amount of delay based on an assigned code, and output them.
• An optical combining section 33 combines optical delayed signals outputted from the first through N' th delay waveguides 32-1 through 32-N, and outputs a resultant signal.
• an optical signal delayed for each wavelength and subjected to spread spectrum is outputted.
• the wavelength separation section 41 separates an input optical signal subjected to spread spectrum into different wavelengths, and outputs them.
• the first through N'th delay waveguides 42-1 through 42-N delay optical signals inputted thereto by an amount of delay based on an assigned code, and output them.
• An optical combining section 43 combines optical delayed signals outputted from the first through N'th delay waveguides 42-1 through 42-N, and outputs a resultant signal .
• an optical signal delayed for each wavelength and subjected to spread spectrum is subjected to inverse spread spectrum and outputted.
• delay waveguides including a photonic crystal are used, and therefore it is possible to provide a compact optical functional circuit which eliminates the need to change the layout thereof and ensures a high productivity rate.
• the wavelength separation section 31 or 41 may include a delay offset section for applying a portion of a delay that is assigned to the first through N'th delay waveguides 32-1 through 32-N or 42-1 through 42-N . This makes it possible to reduce the number of photonic crystals included in the first through N ' th delay waveguides 32-1 through 32-N or 42-1 through 42-N.
• the delay offset section may use the above-described photonic crystal to apply the delay.
• the wavelength separation section 41 may be replaced with an optical splitting section, and delay waveguides are used such that each delay waveguide transmits therethrough only a wavelength different from those transmitted through other delay waveguides.
• the optical code divisionmultiplexing circuit and/orthe optical code division demultiplexing circuit mayincludeMdelaywaveguides which include an optical switching section capable of switching whether to output or not.
• delay waveguides are assigned with their respective amounts ofdelaywhich are different integralmultiples, for example.
• the optical switching section switches whether to output or not in accordance with the M-bit code. For example, an m'th bit of the code is "1", an optical switching section of a delay waveguide having assigned thereto an amount of delay of mx ⁇ performs switching so as to output an optical signal having been delayed.
• the optical code division multiplexing circuit is able to output an optical signal subjected to spread spectrum in accordance with a code pattern. Also, the optical code division demultiplexing circuit is able to subject an optical signal, which has been subjected to spread spectrum, to inverse spread spectrum in accordance with a code pattern.
• FIG. 7 is a block diagram showing a structure of an optical time division multiplexing circuit 5 according to the third embodiment of the present invention. The circuit shown here performs time division multiplexing on an N-channel optical data signal.
• the optical time division multiplexing circuit 5 includes a clock generation section 51, a clock splitting section 52, first through N ' th clock combining sections 53-1 through 53-N, first through N'th delay waveguides 54-1 through 54-N, and an optical combining section 55.
• An m' th delay waveguide 54-m (where m is a natural number from 1 to N) includes one or more photonic crystals 121 arranged in series.
• the optical time division multiplexing circuit 5 receives first through ' th optical data signals which are optical signals .
• the clock generation section 51 generates a predetermined optical clock signal.
• the clock splitting section 52 splits the clock signal into N clock signals, and outputs them.
• An m'th clock combining section 53-m synchronizes an m'th optical data signal with the optical clock signal outputted from the clock splitting section 52 to combine the m' th optical data signal with the optical clock signal, and outputs a resultant signal.
• An m'th delay waveguide 54-m applies a delay of mx ⁇ , which is an integral multiple (m times) of the minimum amount of delay of ⁇ , to the optical signal outputted from the m'th clock combining section 53-m, and outputs a resultant signal. Note that the photonic crystal 121 included in the first through N'th delay waveguides 54-1 through 54-N are the same as those described in the first embodiment.
• FIG. 8 is a block diagram showing a structure of an optical time division demultiplexing circuit 6 according to the third embodiment of the present invention.
• the circuit shown here demultiplexes a data signal subjected to optical time division multiplexing into N channels.
• the optical time division demultiplexing circuit 6 includes a clock generation section 61, a clock splitting section 62, first through N ' th clock combining sections 63-1 through 63-N, first through N'th delay waveguides 64-1 through 6 ' 4-N, and an optical splitting section 65.
• Anm'th delay waveguide 64-m (where m is a natural number from 1 to N) includes one or more photonic crystals 121 arranged in series.
• the optical splitting section 65 receives an input optical signal subjected to optical time division multiplexing.
• the optical splitting section 65 splits the received optical signal into N optical signals.
• An m'th delay waveguide 64-m applies a delay of (N-m+1) x ⁇ , which is an integral multiple ( (N-m+1) times) of the minimum amount of delay of ⁇ , to anra'th one of the N split optical signals, and outputs a resultant signal.
• the photonic crystals 121 included in the first through N'th delay waveguides 64-1 through 64-N are the same as those described in the first embodiment.
• the m'th optical data signal on the transmitter side is delayed by (N+l) ⁇ ⁇ by the time it is outputted from the m'th delay waveguide 64-m.
• the optical signal outputted from the m'th delay waveguide 64-m is inputted to the m'th clock combining section 63-m. Since the first through N'th clock combining sections 63-1 through 63-N receive an optical signal delayed by (N+l) x ⁇ , first through N'th optical data signals are simultaneously inputted to the first through N'th clock combining sections 63-1 through 63-N.
• the first through N'th clock combining sections 63-1 through 63-N are able to simultaneously reproduce the first through N ' th optical data signals based on N optical clock signals obtained by the clock splitting section 62 splitting the optical clock signal derived from the clock generation section 61.
• delay waveguides including a photonic crystal are used, and therefore it is possible to provide a compact optical functional circuit which eliminates the need to change the layout thereof and ensures a high productivity rate. Note that if the optical time division demultiplexing circuit
• FIG. 9 is a block diagram showing a structure of an optical phase control circuit 7 for arrayed antennas according to the fourth embodiment of the present invention.
• the optical phase control circuit 7 for arrayed antennas includes a radio-to-optical conversion section 71, an optical splitting section 72, first through N'th delay waveguides 73-1 through 73-N, first through N'th optical-to-radio conversion sections 74-1 through 74-N, and first through N'th array antennas 75-1 through 75-N.
• a delay waveguide 73-m (where m is an integer from 1 to N) includes photonic crystals 121 arranged in series.
• the radio-to-optical conversion section 71 converts an incoming radio signal into a radio-over-optical signal.
• the optical splitting section 72 splits the radio-over-optical signal into N optical radio signals, and outputs them.
• the first through N'th delay waveguides 73-1 through 73-N which are connected to the optical splitting section 72 in association with output terminals thereof, output delayed signals obtained by causing a delay on the radio-over-optical signals inputted from the optical splitting section 72, so as to cause phases of the radio-over-optical signals to differ from each other.
• amounts of delay are assigned to the first through N'th delay waveguides 73-1 through 73-N such that the phase of a radio signal to be outputted differs for each of the array antennas 75-1 through 75-N.
• the amount of delay is shifted in increments of a time period of ⁇ such that the phases of the radio signals outputted from the array antennas 75-1 through 75-N differ by units of 360/N degrees.
• N amounts of delay of ⁇ , 2 ⁇ ⁇ , 3 ⁇ , and 4 ⁇ ⁇ are set such that phases of radio signals to be outputted differ by units of 90 degrees.
• the photonic crystals 121 included in the first through N'th delay waveguides 73-1 through 73-N are the same as those described in the first embodiment.
• An m'th optical-to-radio conversion section .74-m is connected to a corresponding m'th delay waveguide 73-m so as to convert a radio-over-optical signal outputted from the m'th delay waveguide 73-m into a radio signal.
• An m'th array antenna 75-m is connected to a corresponding optical-to-radio conversion section 74-m so as to emit the radio signal outputted from the m'th optical-to-radio conversion section 74-m toward a direction corresponding to the phase of the radio signal.
• radio signals outputted from the first through N'th optical-to-radio conversion sections 74-1 through 74-N are emitted toward spatially different directions.
• the first through fourth delay waveguides 73-1 through 73-4 delay radio signals such ' that their phases differ by units of 90 degrees.
• the first through N ' th array antennas 75-1 through 75-4 emit the radio signals toward directions differing by units of 90 degrees.
• the radio signal inputted to the radio-to-optical conversion section 71 is emitted from the first through N'th array antennas 75-1 through 74-N toward different directions .
• delay waveguides including a photonic crystal are used, and therefore it is possible to provide a compact optical functional circuit which eliminates the need to change the layout, thereof and ensures a high productivity rate.
• a fifth embodiment of the present invention will be described with respect to an optical digital-to-analog conversion circuit which is an optical functional circuit for converting an optical digital-waveform signal into an optical analog-waveform signal, and outputs a resultant signal.
• FIG. 10 is a block diagram showing a structure of an optical digital-to-analog conversion circuit 8 according to the fifth embodiment of the present invention. Described herein is an example of converting a binary optical digital-waveform signal into a multi-level optical analog-waveform signal.
• FIG. 11 is a diagram for explaining exemplary signal flows in the optical digital-to-analog conversion circuit 8.
• the optical digital-to-analog conversion circuit 8 includes an optical splitting section 81, first through N'th delay waveguides 82-1 through 82-N, first through N'th weighting sections 83-1 through 83-N, and an optical combining section 84.
• An m'th delay waveguide 82-m (where m is a natural number from 1 to N) includes photonic crystals 121 arranged in series .
• the optical splitting section 81 splits an incoming optical digital-waveform signal into N optical digital-waveform signals, and outputs them.
• An m'th delay waveguide 82-m applies a delay of mx ⁇ , which is an integral multiple (m times) of the minimum amount of delay of ⁇ , to an optical digital-waveform signal outputted from the optical splitting section 81, and outputs an m'th optical delayed signal.
• the photonic crystals 121 included in the first through N'th delay waveguides 82-1 through 82-N are the same as those described in the first embodiment.
• Anm' th weighting section 83-m is provided so as to correspond to the m' th delay waveguide 82-m, and has a predetermined weighting factor assigned thereto.
• the weighting factor corresponds to an adjustment ratio for amplitude of an incoming optical signal.
• the adjustment ratio can be 1, 1/2, 1/4, or 1/8.
• the m'th weighting section 83-m adjusts the amplitude of an m'th optical delayed signal outputted from the m'th delay waveguide 82-m based on an adjustment ratio assigned thereto, and outputs a resultant signal .
• N optical delayed signals having a predetermined amplitude ratio are outputted from the first through N'th weighting sections 83-1 through 83-N.
• the first delay waveguide 82-1 outputs a first optical delayed signal obtained by causing a delay on an optical digital-waveform signal by ⁇ .
• the second delay waveguide 82-2 outputs a second optical delayed signal obtained by causing a delay on an optical digital-waveform signal by 2 ⁇ ⁇ .
• the N-l'th delay waveguide 82-N-l outputs an N-l'th optical delayed signal obtained by causing a delay on an optical digital-waveform signal by (N-l) ⁇ ⁇ .
• the N' th delay waveguide 82-N outputs an N'th optical delayed signal obtained by causing a delay on an optical digital-waveform signal by Nx ⁇ . That is, the amounts of delay differ from each other, and are integral multiples of the minimum amount of delay of ⁇ .
• the weighting sections adjust amplitudes of their respective optical delayed signals, and each optical delayed signal whose amplitude has been adjusted is inputted to the optical combining section 84.
• the optical combining section 84 combines the N optical delayed signals whose amplitudes have been adjusted and which have been outputted from the first through N ' th weighting sections 83-1 through 83-N, and outputs an optical analog-waveform signal.
• the minimum amount of delay of ⁇ is assumed to be equal to a time period corresponding to one bit
• the sum of all weighted bits appears at a time delayed by Nx ⁇ from the beginning of the optical digital-waveform signal.
• the time delayed by Nx ⁇ from the beginning of the optical digital-waveform signal is referred to as a "time gate" .
• the time gate arrives per time period of Nx ⁇ .
• a circuit which receives an optical analog-waveform signal outputted from the optical digital-to-analog conversion circuit 8 detects an amplitude value of the optical signal at each time gate to recognize anM-bit sequence corresponding to an optical digital-waveform signal.
• the minimum amount of delay of ⁇ is not limited to a time period corresponding to one bit. Intervals between time gates vary in accordance with the minimum amount of delay of ⁇ .
• an optical digital-waveformsignal in which M bits are expressed by a binary optical power, is detected at a predetermined time gate, and converted into an optical analog-waveform signal which represents one bit by a multi-level optical power.
• information represented by the M-bit optical digital-waveform signal is included at time gates having intervals greater than one bit therebetween. Accordingly, information included in an optical digital-waveformsignal havingbeenprocessedat high speed and outputted can be extracted on the receiver side by detecting an amplitude value of an optical analog-waveform signal at a processing speed lower than that on the transmitter side.
• the optical digital-to-analog conversion circuit 8 can be used in a system where a relatively slower reception circuit, which is able to recognize an analog-waveform, receives a faster opticalpulsepatternsignal.
• delay waveguides including a photonic crystal are used, and therefore it is possible to provide a compact optical functional circuit which eliminates the need to change the layout thereof and ensures a high productivity rate.
• a sixth embodiment of the present invention is described with respect to an optical routing circuit based on optical label switching which is an optical functional circuit for processing label information, which is contained in an optical signal and indicates path information, etc. , without converting the optical signal to an electrical signal .
• FIG.12 is a block diagram showing a structure of an optical routing circuit 9 according to the sixth embodiment of the present invention.
• the optical routing circuit 9 includes an optical splitting section 91, a delay waveguide 94, a label recognition section 92, and an optical path switching section 93.
• the delay waveguide 94 includes photonic crystals 121 arranged in series.
• the photonic crystals 121 are the same as those described in the first embodiment.
• an operation of the optical routing circuit 9 shown in FIG. 12 is described.
• the 'optical splitting section 91 splits an input optical signal containing label information into two optical signals, and outputs them.
• the label recognition section 92 recognizes the label information based on one of the two optical signals obtained by the optical splitting section 91.
• the label information includes destination information, path information, etc.
• a method for multiplexing the label, information with data signal can be, but not limited to, subcarrier multiplexing.
• the delay waveguide 94 applies to the other of the two optical signals outputted from the optical splitting section 91 an amount of delay of kx ⁇ which corresponds to a time period required for the label recognition section 92 to recognize the label information, and outputs a resultant signal.
• the optical path switching section 93 has a plurality of output ports, and outputs an optical signal, which has been outputted from the delay waveguide 94, to a predetermined output port in accordance with an instruction based on a label recognized by the label recognition section 92. Since an input signal having been delayed by a time period required for label recognition is inputted to the optical path switching section 93, the optical path switching section 93 is able to switch between paths for the input signal in accordance with a result of recognition by the label recognition section 92. With the above operation, a first data packet affixed with a first label which is contained in the input signal inputted to the optical splitting section 91 is outputted from a first output port of the optical path switching section 93, which corresponds to a desiredpath or destination.
• a second data packet affixed with a second label is outputted from a second output port of the optical path switching section 93, which corresponds to another or destination.
• delay waveguides including a photonic crystal are used, and therefore it is possible to provide a compact optical functional circuit which eliminates the need to change the layout thereof and ensures a high productivity rate.
• the optical path switching section 93 switches between output ports to select an output port from which an optical delayed signal should be outputted, based on a result of recognitionby the label recognition section 92.
• the ' present invention provides a small-sized optical functional circuit which achieves effects of eliminating the need to change the layout of waveguides on a product-by-product basis and ensuring a high productivity rate, and therefore is useful as an optical code division multiplexing circuit.
• the optical functional circuit of the present invention is applicable to an optical code division demultiplexing circuit, an optical time division multiplexing circuit, an optical time division demultiplexing circuit, an optical phase control circuit for arrayed antennas, an optical digital-to-analog conversion circuit, an optical routing circuit having an optical label switching function, etc.

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