JP2012235411A - Optical receiver and optical reception method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical receiver and an optical reception method for securing mounting accuracy sufficient for reception even if a wavelength interval is narrow in demultiplexing of a wavelength multiplex optical signal using a dielectric multilayer film filter and realizing optical demultiplexing.SOLUTION: The optical receiver demultiplexes a plurality of signal light beams (λ1 to λ4) which are transmitted by wavelength multiplex and different in wavelengths into signal light beams of the respective wavelengths to receive them. The receiver includes: an etalon 13 demultiplexing the plurality of signal light beams whose wavelengths are multiplexed into transmission signal light beams (λ1 and λ3) and reflected signal light beams (λ2 and λ4); the dielectric multilayer film filters 15a and 15b demultiplexing the transmission signal light beams and the reflected signal light beams into the plurality of signal light beams different in the wavelengths; light receiving elements (18a to 18d) receiving the plurality of demultiplexed signal light beams with different wavelengths; and amplifiers (19a to 19d) amplifying the signals received by the light receiving elements.

Description

  The present invention relates to an optical receiving apparatus and an optical receiving method for receiving a plurality of signal lights having different wavelengths that have been wavelength-multiplexed and transmitted by demultiplexing the signal lights.

  An optical signal transmitted through an optical fiber by wavelength multiplexing (WDM: Wavelength Division Multiplexing) of a plurality of signal lights having different wavelengths is received after being demultiplexed into the respective wavelengths. Various methods have been proposed to demultiplex light. For example, Patent Document 1 discloses a method of demultiplexing using a difference in optical path length depending on the wavelength, and Patent Document 2 discloses a method of demultiplexing using a diffraction grating (grating). Patent Document 3 discloses a method of demultiplexing using a dielectric multilayer filter.

JP-A-6-18735 JP-A-62-25710 Japanese Patent Laid-Open No. 9-26521

On the receiving device side in the WDM communication of the metro access system, it is necessary to demultiplex a plurality of signal lights having different wavelengths transmitted through an optical fiber or the like into signal lights of each wavelength with low loss.
The demultiplexing method using the difference in optical path length as shown in Patent Document 1, for example, can make a large difference in refractive index between the optical waveguide portion and the surrounding cladding portion in the Si optical waveguide, so that the bending radius can be reduced. Thus, a small duplexer can be configured. However, there is a coupling loss between the optical fiber and the optical waveguide and a propagation loss of the optical waveguide, and there is a problem that the reception sensitivity is lowered when used for long-distance transmission.

In addition, the demultiplexing method using a diffraction grating as disclosed in Patent Document 2 requires a high processing accuracy as the wavelength interval is shortened, which is not practical.
On the other hand, the method using the dielectric multilayer filter disclosed in Patent Document 3 has a feature that the loss is small because it is a demultiplexing due to reflection and transmission, and is widely used in access systems. However, considering the use of a filter that can be configured with low loss in LAN WDM of 25 Gbps × 4 wavelengths, the wavelength interval of received light is narrow and the steep filter characteristics are obtained at the wavelength interval of 800 GHz required by this communication standard. For this reason, there is a problem that a lot of film forming time is required, which is costly and requires strict mounting accuracy.

  The present invention has been made in view of the above-described situation, and is a demultiplexing of a wavelength-multiplexed optical signal using a dielectric multilayer filter, so that sufficient mounting accuracy can be ensured even when the wavelength interval is narrow. An object of the present invention is to provide an optical receiving apparatus and an optical receiving method.

An optical receiver and an optical reception method according to the present invention receive a plurality of signal lights having different wavelengths transmitted by wavelength multiplexing by demultiplexing them into signal lights of the respective wavelengths. An etalon that demultiplexes the transmitted signal light into transmitted signal light and reflected signal light, a dielectric multilayer filter that demultiplexes each of the transmitted signal light and reflected signal light into a plurality of signal lights having different wavelengths, and demultiplexing And a light-receiving element that respectively receives a plurality of signal lights having different wavelengths, and an amplifier that amplifies the signal received by the light-receiving element.
Note that a second etalon that transmits the reflected signal light demultiplexed by the etalon may be provided, and an etalon having an effective temperature coefficient of zero that does not require temperature management is used. May be.

  According to the present invention, even when the wavelength interval of signal light to be wavelength-multiplexed is narrow, a dielectric multilayer filter designed with twice the wavelength interval of signal light can be used, so that the cost can be reduced and high accuracy can be achieved. It becomes possible to easily receive without mounting. Further, the wavelength dependency of the reception sensitivity is eliminated, and noise can be easily removed.

It is a figure explaining an example of the optical receiver by this invention. It is a figure explaining the other example of the optical receiver by this invention. It is a figure explaining the transmittance | permeability and reflectance of incident light by an etalon. It is a figure explaining the relationship between the light transmission intensity of etalon, and a frequency. It is a figure explaining the relationship between the light incident angle to an etalon, and transmission intensity. It is a figure explaining the relationship between the light incident angle to an etalon, and the fluctuation | variation of the resonant frequency interval. It is a figure explaining the temperature dependence of the resonance frequency interval of an etalon. It is a figure explaining the temperature dependence of the transmission intensity of an etalon.

  The outline of the optical receiver of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an example of an embodiment of the present invention. In the figure, 10 is an optical fiber, 11 is a parallel lens, 12 is an electronic cooling module, 13 is an etalon, 14 is a first mirror, and 15a and 15b are dielectrics. 16a and 16b are second mirrors, 17a to 17d are condenser lenses, 18a to 18d are light receiving elements, and 19a to 19d are amplifiers.

  The wavelength-multiplexed optical signal is transmitted through the optical fiber 10 and input to the input unit of the optical receiver. In this optical signal, for example, it is assumed that four signal lights having different wavelengths λ1, λ2, λ3, and λ4 are multiplexed. Here, for the wavelengths λ1 to λ4, for example, a wavelength in the 1300 nm band is used, λ1 is 229.16 THz (1309.14 nm), λ2 is 229.96 THz (1304.58 nm), and λ3 is 230.76 THz (1300. 05 nm), and λ4 is a frequency (wavelength) of 231.56 THz (129.56 nm). That is, the wavelength interval between these signal lights is a narrow frequency interval of 800 GHz.

  The wavelength-multiplexed optical signal from the optical fiber 10 is converted into parallel light by the parallel lens 11, and the etalon 13 mounted on the electronic cooling module 12 has a predetermined angle (for example, 3 ° with respect to the normal of the etalon surface). Angle). Of the plurality of signal lights (λ1, λ2, λ3, λ4) incident on the etalon 13, the signal light that passes through the etalon 13 (for example, λ1 and λ3) is reflected on the etalon surface without being transmitted. It is demultiplexed into signal light (for example, λ2 and λ4). The reflected reflected signal light (λ2, λ4) is reflected by the first mirror 14 in a predetermined direction.

  The transmitted signal light (λ1, λ3) transmitted through the etalon 13 is incident on the dielectric multilayer filter 15a at a predetermined angle (for example, an angle of 10 ° or less where the polarization dependence of the filter is small). The incident transmitted signal light (λ1, λ3) has a predetermined wavelength (for example, λ1) that passes through the dielectric multilayer filter 15a and a predetermined wavelength (for example, λ3) that is reflected without being transmitted. ) And two signal lights. The signal light (λ1) transmitted through the dielectric multilayer filter 15a is condensed by the condenser lens 17a and received by the light receiving element 18a. The reflected signal light (λ3) is reflected by the second mirror 16a, and is received by the light receiving element 18b through the condenser lens 17b.

  The reflected signal light (λ2, λ4) reflected by the etalon 13 and reflected in a predetermined direction by the first mirror 14 is incident on the dielectric multilayer filter 15b at a predetermined angle (for example, an angle of 10 ° or less). Is done. The incident transmitted signal light (λ2, λ4) is a predetermined wavelength (for example, λ2) that passes through the dielectric multilayer filter 15b and a predetermined wavelength (for example, λ4) that is reflected without being transmitted. ) And two signal lights. The signal light (λ2) transmitted through the dielectric multilayer filter 15b is condensed by the condenser lens 17c and received by the light receiving element 18c. The reflected signal light (λ4) is reflected by the second mirror b, and is received by the light receiving element 18d through the condenser lens 17d.

  Each signal light received by the light receiving elements 18a to 18d arranged in an array or individually is converted into an electric signal, inputted to each of the amplifiers 19a to 19d, and amplified to be a predetermined received signal. Signal transmission from the light receiving elements 18a to 18d to the amplifiers 19a to 19d is performed by metal wire.

Compared with the embodiment of FIG. 1, the embodiment shown in FIG. 2 makes the reflected signal light (λ2, λ4) reflected by the etalon surface incident on the second etalon and transmits the transmitted signal light to the dielectric multilayer. Although different in that it is incident on the membrane filter, the other configurations are the same.
That is, the wavelength-multiplexed optical signal from the optical fiber 10 is converted into parallel light by the parallel lens 11 and is incident on the first etalon 13 mounted on the electronic cooling module 12 at a predetermined angle. Of the plurality of signal lights (λ1, λ2, λ3, λ4) incident on the first etalon 13, the signal light (λ1, λ3) that passes through the first etalon 13 is reflected on the etalon surface without being transmitted. Is demultiplexed into signal light (λ2, λ4).

The transmitted signal light (λ1, λ3) transmitted through the first etalon 13 is incident on the dielectric multilayer filter 15a at a predetermined angle. The incident transmitted signal light (λ1, λ3) has a predetermined wavelength (for example, λ1) that passes through the dielectric multilayer filter 15a and a predetermined wavelength (for example, λ3) that is reflected without being transmitted. ) Signal light and two signal lights.
The reflected signal light (λ2, λ4) reflected by the first etalon 13 is reflected in a predetermined direction by the first mirror 14 and is incident on the second etalon 13 ′ at a predetermined angle. The incident reflected signal light (λ2, λ4) is a signal light having a predetermined wavelength (λ2) that passes through the dielectric multilayer filter 15b and a signal light having a predetermined wavelength (λ4) that is reflected without being transmitted. It is demultiplexed into two signal lights.

  Next, functions and the like of the etalon 13, 13 'described above will be described. An etalon is a device with a simple configuration in which reflective resonators are formed by semi-transparent mirrors parallel to each other. It has a function as a narrow-band wavelength filter with high transmission characteristics, and has low wavefront distortion and insertion loss. Are known.

  FIG. 3 is a diagram schematically showing the relationship between the transmitted light and reflected light of the light incident on the etalon and its frequency (wavelength). The horizontal axis represents frequency (Hz) and the vertical axis represents the relative transmittance of incident light. The reflectivity is shown. The etalon has a characteristic in which light transmittance peaks periodically occur at regular intervals as indicated by a solid line X with respect to the horizontal frequency axis, and the above period is determined by the interval between the transmission mirrors. The frequency interval at which this transmittance peak occurs is said to be the free spectral region (commonly called FSR). Further, as shown by the dotted line Y, the light reflected from the etalon surface is periodically generated at equal intervals in the frequency region where the light does not pass, similarly to the peak value of the transmittance.

  One central wavelength of the transmitted light X having the peak transmittance of the etalon 13 is set to, for example, 230.76 THz (130.005 nm) matched with the wavelength of the signal light λ3, and the FSR is set to 1600 GHz. The central wavelength at which the transmittance reaches a peak is the same as the signal light λ1. Further, the central wavelength in the frequency region of the reflected light is the same as the signal lights λ2 and λ4 that are separated from the signal lights λ1 and λ3 by 800 GHz.

  As described above, by setting the transmission characteristics of the etalon 13 in FIGS. 1 and 2, the signal light λ1 among the plurality of signal lights (λ1, λ2, λ3, λ4) incident on the etalon 13 from the optical fiber 10 is set. And λ3 pass through the etalon 13 and enter the dielectric multilayer filter 15a at the next stage. On the other hand, the signal lights λ2 and λ4 are reflected by the surface of the etalon 13, and then enter the dielectric multilayer filter 15b at the next stage by the first mirror 14.

  In the embodiment of FIG. 2, the etalon 13 is a first etalon, and a second etalon 13 ′ is installed separately from this, and the reflected signal light reflected by the first etalon 13 by the first mirror 14. λ2 and λ4 are incident. The second etalon 13 ′ is similar to the first etalon 13, but is designed so that the central wavelength at which the transmittance reaches a peak is the same wavelength as the signal light λ2 or λ4 and the FSR is 1600 GHz. Is done. As a result, only the signal lights λ2 and λ4 are transmitted, and other peripheral noise signals are cut. As a result, wavelength noise other than the reception wavelength can be removed, and noise resistance can be improved.

  FIG. 4 is a diagram showing the transmittance characteristics when the end face reflectance of the etalon is 0.9. When the permissible range is a transmission intensity of 90% in this transmittance characteristic (peak frequency is 230.65 THz), the permissible error range of the FSR is about 8 GHz. Assuming that the wavelength of the signal light is about 5 GHz due to a shift in the transmission wavelength of the laser diode, etc., the allowable error range (ΔFSR) of the FSR on the etalon side is 3 GHz (± 1.5 GHz). Therefore, the characteristic variation of the etalon is designed to be within this allowable error range.

  FIG. 5 is a diagram showing the relationship between the incident angle of incident light with respect to the normal of the etalon surface and the transmission intensity. For example, the transmission intensity when the incident angle is designed at 3 ° is used as a reference, and the transmission intensity due to the variation of the incident angle It shows a change. According to this figure, if the permissible fluctuation amount of the transmission intensity is within 10%, the mounting accuracy (tilt angle) of the etalon is ± 0.4 °.

  FIG. 6 is a diagram showing a relationship between an allowable error range (ΔFSR) and an incident angle of incident light with respect to a normal of the etalon surface. Assuming that the incident angle of the incident light on the etalon is allowed to be ± 0.4 ° according to FIG. 5, the variation of ΔFSR is 0.6 GHz, and the allowable error range described in FIG. 4 (ΔFSR: 1.5 GHz or less) The design value within the range can be satisfied.

In general, synthetic quartz or the like is used as an etalon material, but the refractive index and volume fluctuate due to temperature changes. For this reason, the allowable error range (ΔFSR) of the etalon is temperature dependent.
FIG. 7 shows the variation characteristics of ΔFSR in which the optical receiver is used in the range of −5 ° C. to 85 ° C. with reference to ΔFSR of transmitted light at a room temperature of 25 ° C. According to this figure, ΔFSR fluctuates in the range of −0.4 GHz to 0.7 GHz. However, even when combined with the incident light error of 0.6 GHz explained in FIG. 4, the allowable error range (ΔFSR: The design value of 1.5 GHz or less) can be satisfied.

  The transmission intensity of the transmitted light of the etalon also shows temperature dependency as shown in FIG. FIG. 8 shows the transmission intensity variation characteristic that the optical receiver is used in the range of −5 ° C. to 85 ° C. based on the transmission intensity of transmitted light at room temperature of 25 ° C. According to this figure, the transmission intensity of the etalon varies about 10% in the above temperature range, and when combined with the allowable value of 10% for the mounting accuracy of the etalon described above, it varies by 20%. Therefore, it is preferable to control the temperature by mounting the etalon in an electronic cooling module or the like made of a Peltier element.

  However, temperature control can be omitted by using an etalon that does not depend on temperature. Note that an etalon having no temperature dependence can be realized, for example, by combining a etalon material with a positive and negative temperature fluctuation. In addition, the temperature characteristics of the etalon may be stored in a memory, and a wavelength may be locked by adding a correction amount at the operating temperature.

  As described above, when receiving a wavelength multiplexed optical signal with a narrow wavelength interval, the signal light demultiplexed by the transmitted signal light and the reflected signal light by the etalon is demultiplexed by the dielectric multilayer filter, thereby receiving low loss. A system can be constructed. For example, even with signal light having a narrow wavelength interval of 800 GHz, by using an etalon, the wavelength interval of the transmitted signal light is doubled to 1600 GHz. By widening the wavelength interval between the two signal lights to be demultiplexed, it is possible to provide a sufficient mounting accuracy of the dielectric multilayer filter, and it is possible to perform reception with low loss and no wavelength dependency. Further, since the signal light passes through the etalon, wavelength noise other than a predetermined reception wavelength is removed, and it is possible to improve resistance to noise.

DESCRIPTION OF SYMBOLS 10 ... Optical fiber, 11 ... Lens, 12 ... Electronic cooling module, 13 ... Etalon, 14 ... 1st mirror, 15a, 15b ... Dielectric multilayer filter, 16a, 16b ... 2nd mirror, 17a-17d ... Collection Optical lenses, 18a to 18d, light receiving elements, 19a to 19d, amplifiers.

Claims (4)

An optical receiver that demultiplexes and receives a plurality of signal lights having different wavelengths transmitted by wavelength multiplexing, into signal lights of respective wavelengths,
An etalon for demultiplexing the plurality of wavelength-multiplexed signal lights into a transmitted signal light and a reflected signal light, and demultiplexing each of the transmitted signal light and the reflected signal light into a plurality of signal lights having different wavelengths. An optical receiver comprising: a dielectric multilayer filter; a light receiving element that receives each of the plurality of signal lights with different wavelengths; and an amplifier that amplifies the signal received by the light receiving element. .   The optical receiver according to claim 1, further comprising a second etalon that transmits the reflected signal light demultiplexed by the etalon.   3. The optical receiver according to claim 1, wherein the etalon has an effective temperature coefficient of zero that does not require temperature management. 4. An optical reception method for receiving a plurality of signal lights having different wavelengths transmitted by wavelength multiplexing by demultiplexing into a signal light of each wavelength,
After the plurality of wavelength-multiplexed signal lights are demultiplexed into transmitted signal light and reflected signal light by an etalon, each of the transmitted signal light and the reflected signal light is divided into a plurality of different wavelengths by a dielectric multilayer filter. An optical receiving method comprising: demultiplexing the received signal light into a plurality of signal lights, receiving each of a plurality of the separated signal lights having different wavelengths by a light receiving element, and amplifying the received signal.

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