JP5015822B2 - Optical demultiplexer - Google Patents

Description

  The present invention relates to an optical device for communication, and more particularly, to an optical demultiplexer used for separating subcarriers in a multicarrier modulation scheme using a plurality of subcarriers.

  In recent years, in response to the need for higher speed and larger capacity of optical transmission, wavelength division multiplexing (WDM) optical transmission technology with a very large capacity of several T (tera) b / s to several tens of Tb / s has become popular. Researched and developed. In such super large capacity transmission, a high bit rate such as 40 G (giga) b / s or 100 Gb / s per wavelength channel is required. In general, if the modulation symbol rate (modulation code transmission rate) is increased for higher speed, the dispersion tolerance deteriorates drastically and the spread of the signal spectrum also increases, so multi-level / multiplexing that increases the bit amount per symbol Is one of the key technologies.

  An orthogonal frequency division multiplexing (OFDM) modulation / demodulation scheme is promising as a multiplexing technique with high dispersion tolerance and high bandwidth utilization efficiency. This system is multicarrier transmission using N orthogonal subcarriers, and has been developed and introduced mainly in wireless communication and satellite broadcasting. By using this method as a modulation means in optical transmission, it is possible to ease restrictions on transmission distance and capacity caused by chromatic dispersion and polarization mode dispersion of an optical fiber as a transmission path. This is because the modulation symbol rate for the same transmission bit rate can be reduced to 1 / N as compared with the case of transmission with a single carrier.

  As a means for OFDM-modulating an optical signal, a method of driving an optical modulator using an electrically generated OFDM signal, as shown in Non-Patent Document 1, can be considered first. In this case, the configuration of the optical system is simple, but it is necessary to have about N times the modulation symbol rate as the band of the driving electric circuit, and eventually the band of the electric system becomes a bottleneck (bottle) with respect to the transmission bit rate.

  On the other hand, in the “All-Optical (all-optical) OFDM” modulation system as proposed in Non-Patent Document 2, as shown in FIG. Then, the subcarrier light is demultiplexed by using an optical demultiplexer 507, data modulated by modulators 512 and 513, and then multiplexed by a coupler (optical multiplexer) 518 for transmission. Output to the road. In this case, the optical system is more complicated than the above-described means disclosed in Non-Patent Document 1, but the band required for the drive electric circuit (not shown) of each modulator 512, 513 is about the modulation symbol rate. The restriction on the transmission capacity due to the bandwidth of the circuit is relaxed. In particular, in the method using two subcarriers as in Non-Patent Document 2, the optical system configuration of the transmitter / receiver is relatively simple, and the effect of reducing the modulation symbol rate and the required bandwidth to the electrical system to ½. Therefore, it is promising as a method for realizing high-speed optical transmission of 100 Gb / s class per wavelength channel at a relatively low cost.

SLJansen, I. Morita, N. Takeda, and H. Tanaka, “20-Gb / s OFDM Transmission over 4,160-km SSMF Enabled by RF-Pilot Tone Phase Noise Compensation,” Proc. Of OFC / NFOEC2007, paper PDP15, 2007. A. Sano, H. Masuda, E. Yoshida, T. Kobayashi, E. Yamada, Y. Miyamoto, F. Inuzuka, Y. Hibino, Y. Takatori, K. Hagimoto, T. Yamada, and Y. Sakamaki, “ 30 × 100-Gb / s all-optical OFDM transmission over 1300 km SMF with 10 ROADM nodes, ”Proc. Of ECOC2007, paper PD1.7, 2007. K. Jinguji, N. Takato, A. Sugita, and M. Kawachi, “Mach-Zehnder interferometer type optical waveguide coupler with wavelength-flattened coupling ratio,” Electron. Lett., Vol. 26, No. 17, pp. 1326 -1327, 1990.

Now, in the All-Optical OFDM modulation system as described above, a mixture of optical carriers (see 506 in FIG. 5) at the time of entering a modulator that performs data modulation causes reception sensitivity deterioration due to coherent crosstalk. Is a problem. Therefore, a high extinction ratio is required for an optical demultiplexer that demultiplexes subcarrier light. Here, the extinction ratio refers to, for example, the f ₁ component output from port 1 and the output light from port 2 when subcarrier f ₁ is output from port 1 and f ₂ is output from port 2. It is defined as the light intensity ratio with the f ₁ component to be mixed. On the other hand, considering application to a WDM network, from the viewpoint of reducing manufacturing and operating costs, optical components other than the light source can be used for different wavelength channels, and adjustments according to the wavelength can be made. It is desirable that it is unnecessary.

FIG. 6 shows a structure of an optical demultiplexer 507 used as an optical demultiplexer for separating _two subcarriers f ₁ and f ₂ in the conventional example shown in Non-Patent Document 2. Here, a single Mach-Zehnder optical interferometer (MZI) comprising two 50% couplers 61 and 62 and two arms having different lengths connecting them is used. In this conventional example, the WDM channel interval is 100 GHz, the subcarrier interval is 25 GHz, and the MZI free frequency interval (FSR) is designed to be 50 GHz, which is twice the frequency interval (= modulation symbol rate) between subcarriers. ing.

As illustrated in FIG. 7 (A), the center of the extinction band against the through-side output port 1a of FIG. 6 coincides with f _2, also matches the center of the extinction band and f ₁ for the cross-side output port 1b of FIG. 6 By adjusting so, f ₁ and f ₂ can be separated at the highest extinction ratio and output from port 1 (508 in FIG. 5) and port 2 (510 in FIG. 5), respectively. In addition, since the subcarrier spacing is an integral fraction of the channel spacing, adjusting the optical demultiplexer to obtain the maximum extinction ratio in one channel can provide the best extinction ratio in other channels. Adjustment according to the channel is unnecessary.

However, when the subcarrier interval is not an integral fraction of the channel interval, for example, when transmission is performed at a subcarrier interval of 30 GHz with a channel interval of 100 GHz, the following problem occurs. That is, in the single MZI as described above, as shown in FIG. 7B, if the FSR is set to 60 GHz which is twice the subcarrier interval, the transmission spectrum of each output port for a certain channel. of the bottom but may be adjusted to be f _1, f ₂ respectively, for this case the bottom of the transmission spectrum for remote channel therefrom by an arbitrary n will also shift n × 20 GHz to sub-carrier frequency, Adjustment according to the channel becomes essential.

Further, as shown in FIGS. 8A and 8B, if the FSR of the optical demultiplexer is set to 50 GHz which is an integral fraction of 100 GHz in accordance with the channel interval, adjustment according to the channel is unnecessary. However, the bottom of the transmission spectrum of each output port cannot be adjusted to f ₁ and f ₂ at the same time, and a sufficient extinction ratio cannot be obtained. Assuming that the maximum extinction ratio of MZI is 25 dB, the center of the extinction band is f ₁ +2.5 GHz at the through-side output port 1a and the cross-side output port 1b so that the same extinction ratio is obtained for each subcarrier. In the case of adjusting to f ₂ −2.5 GHz in FIG. 1, the extinction ratio for each subcarrier can be obtained only about 16 dB.

  The present invention has been made in view of the prior art as described above. The purpose of the present invention is to divide the subcarrier interval by an integral number of the WDM channel interval in WDM transmission using the 2-subcarrier All-Optical OFDM modulation scheme. An object of the present invention is to provide an optical demultiplexer which can separate subcarriers with a high extinction ratio even when the number is not 1, and does not require adjustment according to the channel.

In order to achieve the above object, an optical demultiplexer according to the present invention includes a first optical demultiplexing element, a second optical demultiplexing element, and a third optical demultiplexing that have the same free frequency interval (FSR) of transmission spectrum. And the second optical demultiplexing element and the third optical demultiplexing element are respectively connected to different output ports of the first optical demultiplexing element, and the FSR is a wavelength multiplexed (WDM) signal. a an integer fraction of the adjacent channel spacing frequency, the separating multiple subcarriers a having a FSR following frequency interval (Delta] f) outputting the WDM signal channels, the FSR is a non-integer multiple of the Delta] f characterized in that there.

  In addition, the first optical demultiplexing element, the second optical demultiplexing element, and the third optical demultiplexing element are each composed of two optical couplers and two arms having different lengths connecting the optical couplers. It is a Mach-Zehnder optical interferometer made of a waveguide.

  The center frequency of the extinction band in at least one output port of the second optical demultiplexing element is such that the extinction band in the output port of the first optical demultiplexing element connected to the second optical demultiplexing element. The center frequency of the extinction band at at least one output port of the third optical demultiplexing element is different from the center frequency of the third optical demultiplexing element so that the output of the first optical demultiplexing element connected to the third optical demultiplexing element It can be characterized by being different from the center frequency of the extinction band at the port.

  The center frequency of the extinction band in at least one output port of the second optical demultiplexing element is such that the extinction band in the output port of the first optical demultiplexing element connected to the second optical demultiplexing element. The first optical demultiplexing element connected to the third optical demultiplexing element is different from the center frequency of the third optical demultiplexing element by the center frequency of the extinction band at at least one output port of the third optical demultiplexing element Is different from the center frequency of the extinction band at the output port by −δf, and | δf | is the free frequency interval (FSR) of the first optical demultiplexing device, the second optical demultiplexing device, and the third optical demultiplexing device. It can be characterized by being smaller than 1/2 of the above.

  Further, the optical demultiplexer is one or a plurality of optical demultiplexers for demultiplexing the subcarrier light in an optical communication optical device of a multicarrier modulation method using a plurality of subcarriers. can do.

  With the configuration as described above, according to the optical demultiplexer of the present invention, adjustment according to the channel can be made unnecessary by setting the FSR to be an integral fraction of the channel interval, and at the same time, the optical demultiplexing. There is a remarkable effect that the two subcarriers can be separated with a high extinction ratio by the two-stage element. The same effect can be obtained even if the number of subcarriers is two or more.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 shows a circuit configuration of the optical demultiplexer according to the first embodiment of the present invention. This optical demultiplexer has a configuration using three 50% couplers and three asymmetric Mach-Zehnder interferometers (MZI) composed of two arm waveguides having different lengths connecting them. The optical demultiplexer of the present invention described below is replaced with, for example, the conventional optical demultiplexer 507 shown in FIG.

As shown in FIG. 1, an input port to which the two multiplexed sub-carrier light is input (not shown), the first asymmetric Mach-Zehnder interferometer First (hereinafter, MZI ₁₎ is connected to, the MZI ₁ A second asymmetric Mach-Zehnder interferometer (hereinafter referred to as MZI ₂ ) and a third asymmetric Mach-Zehnder interferometer (hereinafter referred to as MZI ₃ ) are cascade-connected to the through-side output port 1a and the cross-side output port 1b, respectively. The cross-side output port 2b of MZI ₂ and the cross-side output port 3b of MZI ₃ are connected to output ports (not shown) that output the separated subcarriers f ₁ and f ₂ , respectively.

Here, the frequency interval Δf between the _two subcarriers f ₁ and f ₂ is 30 GHz, and is located at −15 GHz and +15 GHz on the frequency axis with respect to the carrier f ₀ , respectively. It is designed on the assumption that it is applied to WDM with a channel spacing of 100 GHz.

The FSRs (free frequency intervals) of MZI ₁ , MZI ₂ , and MZI ₃ are all 50 GHz. In general, an MZI FSR is related to two waveguides having different lengths connecting two couplers, where the group refractive index is N and the waveguide length difference is ΔL.

Given in. Where c is the speed of light in vacuum. In this example, a quartz glass waveguide of N = 1.488 formed on a silicon substrate is used as the waveguide. Therefore, ΔL = c / (N · FSR) = 4029.5 μm. The present invention can be applied regardless of the type of the waveguide, and other examples of the waveguide include a silicon waveguide, a polymer waveguide, and a LiNbO ₃ (lithium niobate) waveguide.

  The maximum extinction ratio of one stage of MZI is such that the coupler branching ratio is exactly 50%, no polarization rotation occurs in the circuit, or the waveguide birefringence is zero, and no radiation loss occurs. Assuming ideal conditions, it becomes infinite. However, since there are actually manufacturing errors, stress in the circuit, bending of the waveguide, etc., these ideal conditions are not satisfied. For example, in a quartz glass waveguide, the maximum extinction ratio of one stage of MZI is about 25 dB. There are often. In this example, a typical circuit is used, and the maximum extinction ratio of one stage of MZI is 25 dB.

  A multimode interferometer (MMI) is used as the 50% coupler. In addition, for example, a waveguide-type wavelength-independent coupler (WINC) as shown in Non-Patent Document 3 may be used as the 50% coupler. Since any coupler can obtain a flat branching ratio characteristic with respect to the wavelength, it is suitable for use in a wide band.

In the present example, it is adjusted so as to cross the side transmission spectrum of the through side transmission spectrum and MZI ₂ of MZI ₁ match. Further, the cross-side transmission spectrum of MZI _{1 and} the cross-side transmission spectrum of MZI ₃ are adjusted to coincide with each other. As an adjustment method, a method may be used in which a heater (not shown) is loaded on each MZI arm waveguide and the waveguide is heated to finely adjust the optical path length of each arm by the thermo-optic effect. A method of finely adjusting the optical path length by irradiating each arm with ultraviolet light to change the refractive index may be used. In any method, since ΔL (waveguide length difference) is changed, strictly speaking, the FSR also changes due to the adjustment. However, if the amount of movement of the spectrum is about several tens of GHz, the FSR is kept substantially constant. The spectrum can be translated on the frequency axis.

2A and 2B show transmission spectra when light is input to the MZI ₁ input port of FIG. 1 and output from the MZI ₂ cross-side output port 2b and the MZI ₃ cross-side output port 3b. Is shown. The transmission spectra output from the cross-side output ports 2b and 3b are FSR = 50 GHz and a maximum extinction ratio of 50 dB, respectively.

As shown in the enlarged view of FIG. 2 (B), the former center of the extinction band (spectrum when the output from 2b) is spectrum when the sub-carrier f ₂ + and 5GHz shifted, from the latter (2b output The center of the extinction band in FIG. 8 is shifted from the subcarrier f ₁ by −5 GHz, but the extinction ratio with respect to the subcarrier is 31 dB, which is sufficiently larger than the conventional 16 dB shown in FIG. Further, since the FSR of each spectrum is 50 GHz, equivalent characteristics can be obtained without changing the adjustment state of each MZI even if the wavelength used is switched to another channel on the 100 GHz grid.

(Second Embodiment)
The circuit configuration of the second embodiment of the present invention is the same as that of the first embodiment shown in FIG. 1, and the frequency interval of the input subcarriers is also 30 GHz, and the WDM channel interval is also 100 GHz. Adjustment is different. 3A and 3B, light is input to the input port of MZI ₁ in FIG. 1, and the cross-side output port 2b of MZI _{2 and} the cross-side output of MZI ₃ in the second embodiment of the present invention. The transmission spectrum when output from the port 3b is shown.

The FSR of each MZI is 50 GHz. In this example, the transmission spectrum of MZI is shifted between the first stage and the second stage. That is, as shown in the enlarged view of FIG. 3 (B), the through side transmission spectrum of the MZI ₁ to the cross-side transmission spectrum of the MZI ₂ has been delta] f = + 2.5 GHz parallel shift on the frequency axis, MZI ₁ The cross-side transmission spectrum of MZI ₃ is shifted by −δf = −2.5 GHz on the frequency axis. As a result, the extinction ratio with respect to the subcarrier is 40 dB, which is sufficiently larger than the conventional 16 dB shown in FIG. 8B, and the FSR of each spectrum is 50 GHz. Even if the channel is switched, the same characteristics can be obtained without changing the adjustment state of each MZI.

FIG. 4 shows the subcarrier extinction ratio with respect to the change in the speckle shift amount δf (first and second stage MZI spectrum shift amounts) in the configuration of this embodiment in which each MZI has an FSR of 50 GHz and a subcarrier spacing of 30 GHz. The change is plotted. When δf = 2.5 GHz, that is, when the center frequency of the extinction band of MZI ₂ is matched with f ₂ and the center frequency of the extinction band of MZI ₃ is matched with f ₁ , the maximum extinction ratio can be obtained. I understand. In other words, in the present embodiment, the first-stage MZI ₁ roughly separates the subcarriers f ₁ and f ₂ with an equal extinction ratio, and the second-stage MZI ₂ and MZI ₃ are f ₂ and f ₁ , respectively. A high extinction ratio is obtained by selectively cutting mixed components.

(Other embodiments)
In the above, the preferred embodiment of the present invention has been described by way of example. However, the embodiment of the present invention is not limited to the above-described example, and the constituent members thereof are within the scope of the claims. Various modifications such as replacement, change, addition, increase / decrease in number, change in shape design, etc. are all included in the embodiments of the present invention. In addition, the present invention may be applied to a system composed of a plurality of devices, or may be applied to an apparatus composed of one device.

  For example, in the above embodiment, it has been shown that two subcarriers are separated with a high extinction ratio by making MZI into two stages, but the same effect can be obtained even if the number of subcarriers is two or more. It is done. Further, a case where the duplexer according to the present invention is connected in parallel to a multicarrier generation circuit that generates a plurality of carriers is also included in the embodiment of the present invention.

1 is a circuit diagram showing a circuit configuration of an optical demultiplexer according to a first embodiment of the present invention. It is a characteristic view which shows the transmission spectrum of the optical demultiplexer of the 1st Embodiment of this invention, (A) is the whole figure, (B) is the enlarged view of the principal part. It is a characteristic view which shows the transmission spectrum of the optical demultiplexer of the 2nd Embodiment of this invention, (A) is the whole figure, (B) is the enlarged view of the principal part. FIG. 10 is a characteristic diagram showing a relationship between a spectral shift amount and a subcarrier extinction ratio in the circuit of the optical demultiplexer according to the second embodiment of the present invention. It is a block diagram which shows the structure of the transmitter in "All-Optical OFDM" proposed by the nonpatent literature 2. FIG. It is a circuit diagram which shows the circuit structure of the optical demultiplexer of a prior art example. It is a characteristic figure which shows the transmission spectrum of the optical demultiplexer of a prior art example, (A) is the 1st example, (B) is a figure which shows the 2nd example. It is a characteristic view which shows the transmission spectrum of the optical demultiplexer of a prior art example, (A) is the 3rd example, (B) is an enlarged view of the principal part.

Explanation of symbols

MZI ₁ First stage first asymmetric Mach-Zehnder interferometer MZI ₂ Second stage second asymmetric Mach-Zehnder interferometer MZI ₃ Second stage third asymmetric Mach-Zehnder interferometer 1a Through-side output port 1b, 2b 3b Cross-side output port 501 CW light source 504 Multicarrier generation circuit 507 Optical demultiplexer 512, 513 Modulator 518 Coupler

Claims (5)

Comprising a first optical demultiplexing element, a second optical demultiplexing element, and a third optical demultiplexing element having the same free frequency interval (FSR) of the transmission spectrum;
The second optical demultiplexing element and the third optical demultiplexing element are respectively connected to different output ports of the first optical demultiplexing element;
The FSR is a fraction of an adjacent channel spacing frequency of a wavelength division multiplexing (WDM) signal,
The separating a plurality of the subcarrier having a FSR following frequency interval (Delta] f) outputting the WDM signal channels, the FSR is an optical demultiplexer, which is a non-integer multiple of the Delta] f. The first optical demultiplexing element, the second optical demultiplexing element, and the third optical demultiplexing element are each composed of two optical couplers and two arm waveguides having different lengths connecting the optical couplers. The optical demultiplexer according to claim 1 , wherein the optical demultiplexer is a Mach-Zehnder optical interferometer. The center frequency of the extinction band at at least one output port of the second optical demultiplexing element is equal to the center of the extinction band at the output port of the first optical demultiplexing element connected to the second optical demultiplexing element. Unlike frequency,
The center frequency of the extinction band at at least one output port of the third optical demultiplexing element is the center of the extinction band at the output port of the first optical demultiplexing element connected to the third optical demultiplexing element. 3. The optical demultiplexer according to claim 1, wherein the optical demultiplexer is different from the frequency. The center frequency of the extinction band at at least one output port of the second optical demultiplexing element is equal to the center of the extinction band at the output port of the first optical demultiplexing element connected to the second optical demultiplexing element. It differs from the frequency by + δf,
The center frequency of the extinction band at at least one output port of the third optical demultiplexing element is the center of the extinction band at the output port of the first optical demultiplexing element connected to the third optical demultiplexing element. It differs from frequency by -δf,
2. | δf | is smaller than ½ of a free frequency interval (FSR) of the first optical demultiplexing element, the second optical demultiplexing element, and the third optical demultiplexing element. 4. The optical demultiplexer according to any one of items 1 to 3 . The optical demultiplexer is one or a plurality of optical demultiplexers for demultiplexing subcarrier light in a multicarrier modulation optical communication optical device using a plurality of subcarriers. Item 5. The optical demultiplexer according to any one of Items 1 to 4 .

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