JP2008209543A - Optical isolator apparatus and integrated semiconductor optical apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical isolator apparatus which does not use a Faraday rotator and is easily integrated with another semiconductor optical apparatus on the same semiconductor substrate. <P>SOLUTION: Frequency of signal light 14 is converted by a first frequency converter 22, is returned to the original frequency by a second frequency converter 32 and is emitted. In the procedure, a first light frequency selector 26 shields the signal light 14 while transmits second frequency converted light 28 originated from the signal light 14. However, return light 12 generated by reflection of the second frequency converted light 28 has an advancing direction reverse to that of excitation light and the frequency of the return light is not therefore converted by the second frequency converter 32. As the result, the return light 12 is shielded by the first light frequency selector 26. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical isolator device and an integrated semiconductor optical device, and in particular, an optical isolator device and another semiconductor optical device that can be integrated on the same semiconductor substrate as other semiconductor optical devices and the optical isolator device are integrated. The present invention relates to an integrated semiconductor optical device.

  An optical device that includes an optical active element such as a semiconductor laser or a semiconductor optical amplifier and generates or processes an optical signal is a small amount of reflected light, that is, return light generated in the optical device (or in the optical transmission line to which the optical device is connected) This also makes the operation of the optical active element unstable. The optical isolator is for blocking the return light and stabilizing the operation of the optical active element. Therefore, the optical isolator is an indispensable optical component for the optical device. In particular, in an optical device equipped with a semiconductor laser or a semiconductor optical amplifier, an optical isolator is always connected to the light exit port.

  As shown in FIG. 8, the optical isolator 1 includes a pair of optical polarizers 2 and 4 and a Faraday rotator 10. A magnetic field 6 parallel to the traveling direction of light is applied to the Faraday rotator 10.

  In the optical isolator 1, the transmission axis 3 of the first polarizer 2 is set to coincide with the polarization direction 9 of the incident light 8. For this reason, the incident light 8 passes through the first polarizer 2. In the Faraday rotator 10 through which the incident light 8 passes next, the polarization rotation angle is set to 45 °. Therefore, the incident light 8 that has passed through the Faraday rotator 10 has a polarization direction rotated by 45 °. Next, the incident light 8 is incident on the second polarizer 4. The transmission axis 5 of the second polarizer 4 is inclined 45 °. Therefore, the incident light 8 passes through the second polarizer 4 and is emitted from the optical isolator 1.

  The return light 12 generated in the optical device (or the optical transmission line to which the optical device is connected) enters the optical isolator 1 and passes through the second polarizer 4. The return light 12 enters the Faraday rotator 10 and the plane of polarization further rotates by 45 °. Therefore, when passing through the Faraday rotator 10, the polarization plane of the return light 12 is rotated by 90 ° with respect to the first polarization plane of the incident light. That is, the polarization direction 13 of the return light 12 is orthogonal to the transmission axis 3 of the first polarizer 2. For this reason, the return light 12 cannot pass through the first polarizer 2. Therefore, the return light 12 is not emitted from the entrance of the optical isolator 1.

In this way, the optical isolator 1 blocks the return light and stabilizes the operation of the semiconductor optical device connected to the optical isolator 1.
JP 2005-315993 A

  The Faraday rotator 10 constituting the optical isolator 1 is made of a magnetic material such as YIG (yttrium, iron, garnet). Attempts have been made to integrate such semiconductor optical devices and optical isolators by forming such a magnetic material on a semiconductor substrate (Patent Document 1). However, this attempt is still at the research stage.

  Moreover, in order to make the Faraday rotator 10 function, it is necessary to apply a strong magnetic field. In order to generate this magnetic field, it is necessary to form a ferromagnetic metal on the semiconductor substrate. However, if metal is formed on the substrate on which the semiconductor optical device is formed, the optical loss increases and the function of the semiconductor optical device is impaired.

  Thus, it is extremely difficult to integrate the optical isolator comprising the Faraday rotator 10 and the semiconductor optical device on the same semiconductor substrate.

  Accordingly, an object of the present invention is to provide an optical isolator device that does not require a Faraday rotator and can be easily integrated on the same semiconductor substrate with other semiconductor optical devices.

  In order to achieve the above object, according to a first aspect of the present invention, there is provided an optical isolator device including a signal light having a first frequency and a pumping light traveling in the same direction as the signal light and having a second frequency. To generate a first frequency-converted light having a third frequency, and to emit a first output light composed of the signal light, the excitation light, and the first frequency-converted light. The first optical frequency converter and the first outgoing light emitted from the first optical frequency converter are incident, the signal light is blocked, and the second light consisting of the excitation light and the first frequency converted light A first optical frequency selector that emits outgoing light and a second outgoing light that is emitted from the first optical frequency selector are incident, and the excitation light and the first frequency converted light that travel in the same direction are mixed. Generating a second frequency-converted light having a first frequency, A second optical frequency converter made of a nonlinear medium that emits a third outgoing light consisting of an electromotive light and first and second frequency converted light; and a third output emitted by the second optical frequency converter. A second optical frequency selector that emits incident light, emits second frequency-converted light, and blocks the excitation light and the first frequency-converted light; a light entrance that receives the signal light; And a second optical frequency selector transmits the return light reflected from the second frequency converted light and returned to the light output port, and the second optical frequency selector An optical frequency converter transmits the return light transmitted by the second optical frequency selector without mixing with the pumping light, and the first optical frequency selector transmits the second optical frequency converter. The return light is blocked.

  According to the first aspect, since the optical isolator can be configured only by the semiconductor optical device that can be formed on the semiconductor substrate without using the Faraday rotator, the semiconductor optical device and the optical isolator are formed on the same semiconductor substrate. It becomes possible to integrate.

  According to a second aspect of the present invention, in the optical isolator device according to the first aspect, the first and second optical frequency converters are semiconductor optical amplifiers.

  According to the first aspect, since the first and second optical frequency converters are reduced in size, the entire optical isolator device can be reduced in size.

  According to a third aspect, in the optical isolator device according to the first or second aspect, the first and second optical frequency converters, the first and second optical frequency selectors, the light incident port, and the light output port. Are formed on the same semiconductor substrate.

  According to the third aspect, the semiconductor optical device and the optical isolator can be integrated on the same semiconductor substrate without using the Faraday rotator.

  According to a fourth aspect, in the first to third optical isolators, the substrate is a substrate made of indium phosphorus.

  The fifth aspect includes an optical isolator device according to the third or fourth aspect and a semiconductor optical device integrated on the semiconductor substrate and having a light exit opening optically connected to the light entrance. Features.

  According to the present invention, since the optical isolator can be configured only by the semiconductor optical device that can be formed on the semiconductor substrate without using the Faraday rotator, the other semiconductor optical device and the optical isolator can be formed on the same semiconductor substrate. It becomes possible to integrate.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to these embodiments, but extends to the matters described in the claims and equivalents thereof. In addition, the same code | symbol is attached | subjected to the same part and the description is abbreviate | omitted.

(Basic configuration)
FIG. 1 is a conceptual diagram of an optical isolator device according to the present embodiment.

The optical isolator device 11 according to the present embodiment mixes the signal light 14 having the first frequency f ₁ and the pumping light 16 having the second frequency f ₂ that travels in the same direction as the signal light. A non-linear medium (for example, a first frequency converted light 18 having a frequency f ₃ of ₃ and a first output light 20 composed of the signal light 14, the excitation light 16, and the first frequency converted light 18) A first optical frequency converter 22 comprising a semiconductor optical amplifier) is provided.

  Further, in the optical isolator device 11 according to the present embodiment, the first outgoing light 20 emitted from the first optical frequency converter 22 is incident, the signal light 14 is blocked, the pumping light 16 and the first frequency A first optical frequency selector 26 that emits the second outgoing light 24 composed of the converted light 18 is provided.

Further, the optical isolator device 11 according to the present embodiment receives the second emission light 24 emitted from the first optical frequency selector 26, mixes the excitation light 16 and the first frequency converted light 18, and A non-linear medium (a non-linear medium) that generates a second frequency converted light 28 having a _first frequency f ₁ and emits a third outgoing light 30 comprising the excitation light 16 and the first and second frequency converted lights 18 and 28; For example, a second optical frequency converter 32 composed of a semiconductor optical amplifier) is provided.

  In addition, the third outgoing light 30 emitted from the second optical frequency converter 32 is incident, the second frequency converted light 28 is emitted, and the excitation light 16 and the first frequency converted light 18 are blocked. The optical frequency selector 34 is provided.

  Furthermore, the optical isolator device 11 according to the present embodiment includes a light incident port 36 through which the signal light 14 is incident and a light emitting port 38 through which the second frequency converted light is emitted.

  In the optical isolator device 11 according to the present embodiment, the second optical frequency selector 34 transmits the return light 12 that is reflected from the second frequency converted light and returned to the light exit port 38, The optical frequency converter 32 transmits the return light 12 transmitted through the second optical frequency selector 34 without being mixed with the pumping light 16, and the first optical frequency selector 26 performs the second optical frequency conversion. The return light 12 transmitted by the device 32 is blocked.

  By adopting such a configuration, the optical isolator device 11 according to the present embodiment prevents the return light 12 from being emitted from the light incident port 36 and functions as an optical isolator.

In addition, the optical isolator device 11 according to the present embodiment and a semiconductor optical device that can be formed on a semiconductor substrate such as a semiconductor optical amplifier can be used. For this reason, the optical isolator device according to the present embodiment can be easily integrated on a semiconductor substrate.
The configuration shown in FIG. 1 is a configuration in which the signal light 14 and the excitation light 16 are combined in advance and both are incident from the light incident port 36. However, the excitation light 16 may be supplied by a light generating device integrated on the same substrate, for example, a semiconductor laser.

Next, the principle of how the optical isolator function is realized by the above configuration will be described.
(principle)
Hereinafter, the principle of the optical isolator device according to the present embodiment will be described with reference to FIG.

  The operation principle of the optical isolator device in the present embodiment will be described by paying attention to the behavior of the signal light 14.

(1) Outline First, the outline of the operating principle will be described.

As shown in FIG. 1, the frequency of the signal light 14 is first converted from f ₁ to f ₃ by the first optical frequency converter 22 (that is, the signal light 14 is converted to the first frequency converted light. ) Further, the frequency is returned from the frequency f ₃ to the original frequency f ₁ by the second optical frequency converter 32 and emitted from the light exit port 28 (that is, the first frequency converted light is converted into the second frequency converted light. Converted to.)

  Is output as frequency-converted light 28.

  In this process, the signal light 14 is removed by the first optical frequency selector 26. Further, the first frequency converted light 18 and the pump light 16 are removed by the second optical frequency selector 34.

On the other hand, since the return light 12 re-entering the optical isolator device 11 has the same frequency f ₁ as the second frequency converted light 28, the second optical frequency selector 34 can pass therethrough. However, the first optical frequency selector 26 that blocks the signal light 14 having the frequency f ₁ cannot pass therethrough.

Moreover, since the return light 12 travels in a direction opposite to that of the excitation light 16, nor the frequency f ₁ is converted to f ₃ by a second frequency converter 32. That is, the return light 12 is not converted by the second frequency converter 32 into the frequency f ₃ that can be transmitted through the first optical frequency selector 26.

  As a result, the return light 12 cannot pass through the first optical frequency selector 26 and cannot reach the light incident port 36. That is, the return light 12 (or light derived from the return light 12) is not emitted from the incident port 36. Therefore, the optical isolation function shown in FIG. 1 realizes the optical isolation function.

  The above is the outline of the principle of the optical isolator device in the present embodiment. Hereinafter, this principle will be described in detail.

(2) Details First, the signal light 14 having the frequency f ₁ enters the optical frequency converter 22 made of, for example, a semiconductor optical amplifier or the like via the incident port 36. The pumping light 16 having the frequency f ₂ is also incident on the optical frequency converter 22. However, the pumping light 16 is not necessarily incident from the outside. For example, the optical isolator device 11 may include a distributed feedback semiconductor laser and generate it itself. However, the excitation light 16 must travel in the same direction as the signal light 14.
First optical frequency converter 22, FIG. 1 and Table 1, the excitation light 16 of the signal light 14 and the frequency _{f 2} of the frequency _{f 1} by mixing (FWM), the frequency _f 3 (= 2f The first frequency converted light 18 having ₂ −f ₁ ) is generated, and the signal light 14 and the excitation light 16 are emitted together to emit the first frequency converted light 18. Table 1 is a table for explaining the operation of the first and second optical frequency converters 22 and 32.

信号光１４、励起光１６、及び第１の周波数変換光１８からなる出射光２０は、第１の周波数選択器２６に入射する。第１の周波数選択器２６は、図１及び表２に示すように、周波数ｆ_１の信号光１４を遮断し、周波数ｆ_２の励起光１６および周波数ｆ_３の第１の周波数変換光１８透過する。尚、表２は、第１及び第２の光周波数選択器２６，３４の動作を説明する表である。

The outgoing light 20 composed of the signal light 14, the excitation light 16, and the first frequency converted light 18 enters the first frequency selector 26. As shown in FIG. 1 and Table 2, the first frequency selector 26 blocks the signal light 14 having the frequency f ₁ and transmits the pump light 16 having the frequency f ₂ and the first frequency converted light 18 having the frequency f ₃ . To do. Table 2 is a table for explaining the operation of the first and second optical frequency selectors 26 and 34.

第１の周波数選択器２６を透過した励起光１６および第１の周波数変換光１８は、例えば半導体光増幅器からなる第２の光周波数変換器３２に入射する。第２の光周波数変換器３２は、図１及び表１に示すように、同一方向に進行する励起光１６と第１の周波数変換光１８を混合（四光波混合）し、周波数ｆ_１（＝２ｆ_２-ｆ_３）を有する新たな周波数変換光２８を生成する。

The pumping light 16 and the first frequency converted light 18 that have passed through the first frequency selector 26 are incident on a second optical frequency converter 32 made of, for example, a semiconductor optical amplifier. As shown in FIG. 1 and Table 1, the second optical frequency converter 32 mixes the excitation light 16 traveling in the same direction and the first frequency converted light 18 (four-wave mixing), and the frequency f ₁ (= 2f ₂ -f ₃ ) is generated.

That is, the frequency-converted light 28 is obtained by sequentially converting the signal light 14 by the two optical frequency converters 22 and 32. Here, since the pump light used in the optical frequency converters 22 and 32 is the same, the frequency of the second frequency converted light 28 is the same frequency f ₁ as the frequency of the signal light 14.
From the second optical frequency converter 32, the excitation light 16 and both frequency converted lights 18, 28 are emitted. However, as shown in FIG. 1 and Table 2, the second of the first excitation light 16 of the frequency converted light 18 and the frequency f ₂ of the frequency f ₃ by the optical frequency selector 34 is cut off, the same frequency as the signal light 14 Only the second frequency converted light 28 having f ₁ is transmitted through the second optical frequency selector 34.

  The second frequency converted light 28 is emitted from the light exit port 38 of the optical isolator device 11 and enters the optical device (not shown) provided with the optical isolator device 11. The second frequency converted light 28 is reflected inside the optical device or in an optical transmission path to which the optical device is connected, and returns to the light exit port 38 as light 12 again.

The return light 12 is a reflection of the second frequency converted light 28. Therefore, the frequency is the same f ₁ as the second frequency converted light 28. Therefore, the return light 12 passes through the second optical frequency selector 34 (see FIG. 1 and Table 2).

  Next, the return light 12 that has passed through the second optical frequency selector 34 enters the second optical frequency converter 32. However, the return light 12 is not frequency-converted by the second optical frequency converter 32 because the traveling direction of the return light 12 is opposite to that of the excitation light 16 as described below. Therefore, the second optical frequency converter 32 emits only the return light 12 to the first optical frequency selector 26 without generating the frequency converted light based on the return light 12.

The return light 12 emitted from the second optical frequency converter 32 enters the first optical frequency selector 26. Here, the return light 12 having the frequency f ₁ is blocked by the first optical frequency selector 26 (see FIG. 1 and Table 2).

  Therefore, the return light 12 cannot reach the light incident port 36 of the optical isolator device 11. In this way, the function as the optical isolation is realized by the configuration as shown in FIG.

(3) Operation of Optical Frequency Converter Next, the reason why the first and second optical frequency converters 22 and 32 do not frequency convert the return light 12 will be described.

  The first and second optical frequency converters 22 and 32 are configured by a nonlinear medium having a large third-order nonlinear susceptibility (not at least zero), such as a semiconductor optical amplifier. The first and second optical frequency converters 22 and 32 mix incident light (probe light) and strong excitation light (pump light) by a third-order nonlinear effect (four-wave mixing), and convert the frequency converted light. Generate.

  The polarization of the nonlinear medium responds nonlinearly to the electric field of light. Therefore, nonlinear polarization that vibrates at a frequency different from that of the probe light and pump light is excited in the nonlinear medium. Nonlinear polarization that vibrates at such a frequency serves as an excitation source, and light that vibrates at a frequency different from the probe light and pump light, that is, frequency-converted light is generated.

  Under normal operating conditions of the optical frequency converter, the non-linear polarization is uniformly distributed along the traveling direction of the probe light and the pump light. That is, the excitation source of the frequency converted light by nonlinear polarization is distributed along the traveling direction of the probe light and the pump light. When the phases of the light generated by the distributed excitation sources are aligned, the frequency converted light grows as the probe light advances. That is, frequency conversion is realized. Such alignment of light generated by nonlinear polarization is called phase matching.

  When the frequencies of the probe light and the pump light are close, the refractive index dispersion of the nonlinear medium can be ignored. Therefore, in such a case, the phase matching condition can be ignored. For example, in an optical frequency converter composed of a semiconductor optical amplifier, the frequency difference between the probe light and the pump light is only a few tens of nm in terms of wavelength. Therefore, it is not necessary to consider the phase matching condition.

However, this is premised on a normal use state in which the traveling directions of the probe light and the pump light coincide. When the traveling directions of the probe light and the pump light are opposite, the phase matching condition is not satisfied even if there is no difference in refractive index. Therefore, in such a case, frequency converted light is not generated.
In the present embodiment, the traveling directions of the return light 12 and the excitation light 16 are opposite. For this reason, the first and second optical frequency converters 22 and 32 do not frequency convert the return light 12.

(3) Phase matching condition Finally, the reason why the phase matching condition is not satisfied when the traveling directions of the probe light and the pump light are opposite will be described.

  First, in the case where the traveling directions of the probe light and the pump light coincide with each other, what the phase matching condition is will be described in detail based on Maxwell's equations. Next, the reason why the phase matching condition is not satisfied when the traveling directions of the probe light and the pump light are opposite will be described.

  As is widely known, an equation describing the electric field intensity E (z, t) of light is expressed as follows based on Maxwell's equation.

ここで、ｚは、光の進行方向に沿った座標である。また、ｔは時間である。また、Ｄ（ｚ，ｔ）は、電束密度である。μ_０は、真空の透磁率である。

Here, z is a coordinate along the traveling direction of light. T is time. D (z, t) is the electric flux density. μ ₀ is the vacuum permeability.

  The electric flux density can be expressed by the following equation.

ここでε_０は、真空の誘電率である。Ｐは分極である。

Here, ε ₀ is the dielectric constant of vacuum. P is polarization.

  This polarization P (z, t) can be expressed separately as a linear part and a non-linear part with respect to the electric field intensity E (z, t) as shown in the following equation.

ここで、χ_１は電気感受率である。Ｐ_ＮＬ（ｚ，ｔ）は、非線形分極である。式（３）を、式（１）に代入して整理すると、次式のような非線形媒質中での光の伝播方程式が得られる。

Here, χ ₁ is the electrical susceptibility. P _NL (z, t) is nonlinear polarization. By substituting Equation (3) into Equation (1) and rearranging it, an equation of light propagation in a nonlinear medium is obtained as in the following equation.

ここでεは、非線形媒質の誘電率であり、ε＝ε_０＋χ_１である。

Here, ε is a dielectric constant of the nonlinear medium, and ε = ε ₀ + χ ₁ .

Further, the nonlinear polarization P _NL (z, t) can be expressed by the following equation.

四光波混合が起きている時の電界強度Ｅ（ｚ，ｔ）は、ポンプ光Ｅ_ｐ（ｚ，ｔ）、プローブ光Ｅ_ｓ（ｚ，ｔ）、及び周波数変換光Ｅ_ｃ（ｚ，ｔ）の和で表すことができる。すなわち、電界強度Ｅ（ｚ，ｔ）は、次式で表ことができる。

The electric field intensity E (z, t) when four-wave mixing occurs is the pump light E _p (z, t), the probe light E _s (z, t), and the frequency converted light E _c (z, t). Can be expressed as the sum of That is, the electric field strength E (z, t) can be expressed by the following equation.

ここで、ポンプ光Ｅ_ｐ（ｚ，ｔ）を次式で表すことがきる。

Here, the pump light E _p (z, t) can be expressed by the following equation.

ω_ｐおよびｋ_ｐは、夫々ポンプ光の角振動数（＝２π／振動数）および波数（＝２π／波長）である。また、ｉは純虚数である。但し、ω_ｐおよびｋ_ｐは正の実数である。

ω _p and k _p are the angular frequency (= 2π / frequency) and wave number (= 2π / wavelength) of the pump light, respectively. I is a pure imaginary number. However, ω _p and k _p are positive real numbers.

Similarly, the probe light E _s (z, t) can be expressed by the following equation.

ω_ｓおよびｋ_ｓは、夫々プローブ光の角振動数（＝２π／振動数）および波数（＝２π／波長）である。但し、ω_ｓおよびｋ_ｓは正の実数である。

ω _s and k _s are the angular frequency (= 2π / frequency) and wave number (= 2π / wavelength) of the probe light, respectively. However, ω _s and k _s are positive real numbers.

Similarly, the frequency converted light E _c (z, t) can be expressed by the following equation.

ω_ｃおよびｋ_ｃは、夫々プローブ光の角振動数（＝２π／振動数）および波数（＝２π／波長）である。但し、ω_ｃおよびｋ_ｃは正の実数である。

ω _c and k _c are the angular frequency (= 2π / frequency) and wave number (= 2π / wavelength) of the probe light, respectively. However, ω _c and k _c are positive real numbers.

Next, the formulas (7) to (9) are substituted into the formula (6) to obtain the electric field strength E (z, t) in the four-wave mixing state. Next, the obtained E (z, t) is substituted into the left side of the equation (4). The result is the sum of the electric field components oscillating at the three angular frequencies ω _p , ω _s and ω _c .

Here, a component that vibrates at the angular frequency ω _c is extracted, and an assumption such as the following equation is made.

この式は、Ｅ_ｃ（ｚ）が、振動成分exp{i(ω_ｃｔ-ｋ_ｃｔ)}+ exp{-i(ω_ｃｔ-ｋ_ｃｔ)}に比べて穏やかに変化することを意味する。更に、ｋ_ｃ＝ω_ｃμ_０εを考慮すると、式（４）の左辺を近似する次式が得られる。

This equation shows that E _c (z) changes gently compared to the vibration component exp {i (ω _c t−k _c t)} + exp {−i (ω _c t−k _c t)}. means. Further, when k _c = ω _c μ ₀ ε is considered, the following equation that approximates the left side of equation (4) is obtained.

ここで、ｃ．ｃ．は複素共役数を表す。

Where c. c. Represents a complex conjugate number.

Next, consider the right side of equation (4), that is, the nonlinear polarization P _NL (z, t).

The nonlinear polarization P _NL (z, t) is expressed by the equation (5). Since four-wave mixing is a third-order nonlinear effect, attention is focused on the second term on the right side of Equation (5).

Therefore, Expression (6) is substituted into the second term on the right side of Expression (5). Then, the second term (P ⁽³⁾ _NL (z, t)) on the right side of the equation (5) is expressed by the following equation.

但し、以下のような仮定をおく。すなわち、式（１２）では、Ｅ_ｃ（ｚ，ｔ）は無視されている。

However, the following assumptions are made. That is, E _c (z, t) is ignored in Expression (12).

次に、式（１２）に式（７）〜（９）を代入する。ここで、三倍波Ｅ^３ _ｐ（ｚ，ｔ）および Ｅ^３ _ｓ（ｚ，ｔ）は無視する。

Next, Expressions (7) to (9) are substituted into Expression (12). Here, the triple wave E ³ _p (z, t) and E ³ _s (z, t) are ignored.

Since E _p (z, t) >> E _s (z, t), E _p (z, t) · E ² _s (z, t) can be ignored.

  Therefore, the equation (12) becomes the following equation.

第一項は、ポンプ光やプローブ光に比べて３倍近くの振動数をもっている。従って、この項によって励起される光は周波数が高く、殆ど基板や非線形媒質によって吸収してしまう。

The first term has a frequency that is nearly three times that of pump light or probe light. Therefore, the light excited by this term has a high frequency and is almost absorbed by the substrate and the nonlinear medium.

  The third term has the same frequency as the probe light. Therefore, although this term has an effect of amplifying the probe light, it has no effect of generating the frequency converted light.

Therefore, in the nonlinear polarization P ⁽³⁾ _NL (z, t) represented by the equation (14), only the second term can be the excitation source of the frequency converted light.

  Therefore, in the state where the frequency converted light is generated, the following equation is established.

ここで式（４）の右辺に式（１４）を代入し、角周波数ω_ｃ（＝２ω_ｐ−ω_ｓ）で振動する成分を抽出する。抽出した成分は、式（４）の左辺において角周波数ω_ｃ振動する項を表す式（１１）に等しい。従って、次式が得られる。

Here, Expression (14) is substituted into the right side of Expression (4), and a component that vibrates at the angular frequency ω _c (= 2ω _p −ω _s ) is extracted. The extracted component is equal to Expression (11) representing the term that vibrates at the angular frequency ω _{c on} the left side of Expression (4). Therefore, the following equation is obtained.

ここで、式（１６）の両辺を次式で割る。

Here, both sides of the equation (16) are divided by the following equation.

すると式（１５）が成立するので、次式の微分方程式が得られる。

Then, since Expression (15) is established, the following differential equation is obtained.

但し、式（１８）を導く際、次式を考慮する。

However, the following equation is taken into account when deriving equation (18).

ここで、Δｋは次式で定義される変数である。

Here, Δk is a variable defined by the following equation.

次に、式（１８）の微分方程式を解き、周波数変換光の強度のＥ_ｐ（ｚ，ｔ）・Ｅ^＊ _ｐ（ｚ，ｔ）を算出すと次式が得られる。

Next, by solving the differential equation of equation (18) and calculating E _p (z, t) · E ^* _p (z, t) of the intensity of the frequency converted light, the following equation is obtained.

式（２１）の右辺は、よく知られているようにΔｋ＝０の場合に、著しく大きな値となる関数である。従って、ｋ_ｃ＝２ｋ_ｐ-ｋ_ｓが満たされた場合、大きな周波数変換光が得られることになる。一方、Δｋ＝０が成立しない場合、生成される周波数変換光は極めて小さく無視することができる。

As is well known, the right side of the equation (21) is a function that takes a remarkably large value when Δk = 0. Therefore, when k _c = 2k _p -k _s is satisfied, a large frequency converted light can be obtained. On the other hand, if Δk = 0 does not hold, the generated frequency converted light is extremely small and can be ignored.

  Satisfying this condition of Δk = 0 is called phase matching.

Incidentally, ω _c = 2ω _p −ω _s as shown in the equation (15). Moreover, as described above, when the optical frequency converter is composed of a semiconductor amplifier, the refractive index dispersion can be ignored. _Therefore, k c = _2k p -k _s also established. Therefore, Δk = 0 and frequency converted light is generated.

  The above is the principle of frequency conversion by four-wave mixing.

Here, consider the case where the pump light and the probe light travel in opposite directions. In this case, the conditions corresponding to the formula (20) is in replaces the _{k s} at -k _s. That is, when the traveling direction of the probe light is opposite to that of the pump light, Expression (20) becomes as follows.

従って、もはやΔｋ＝０は成立しない。従って、プローブ光の進行方向がポンプ光と逆の場合には、周波数変換光は生成されない。

Therefore, Δk = 0 no longer holds. Accordingly, when the traveling direction of the probe light is opposite to that of the pump light, no frequency converted light is generated.

  That is, when the traveling directions of the probe light and the pump light are opposite, the phase matching condition is not satisfied.

(Embodiment 1)
(1) Configuration This embodiment relates to an optical isolator device that can be integrated on the same semiconductor substrate as other semiconductor optical devices. FIG. 2 is a schematic diagram showing a planar structure of the optical isolator device 11 in the present embodiment.

  As shown in FIG. 2, the optical isolator device 11 in the present embodiment is formed on an InP substrate 42 and is constituted by a plurality of semiconductor optical devices.

The optical isolator device 11 includes a signal light 14 having a _first frequency f ₁ (wavelength 1.55 μm) and a pumping light traveling in the same direction as the signal light and having a _second frequency f ₂ (wavelength 1.57 μm). 16, the first frequency converted light 18 having the _third frequency f ₃ (wavelength 1.59 μm) is generated, and the first frequency converted light 18 including the signal light 14, the excitation light 16, and the first frequency converted light 18 is generated. The first optical frequency converter 22 composed of a semiconductor optical amplifier 44 (nonlinear medium) that emits the emitted light 20 is provided.

  The optical isolator device 11 includes a distributed feedback (DFB) semiconductor laser 43 that generates the pumping light 16. In addition, the optical isolator device 11 includes a multi-mode interference (MMI) coupler 54 that couples the signal light 14 and the pumping light 16 and emits them to the first optical frequency converter 22.

  Also, the optical isolator device 11 receives the first outgoing light 20 emitted from the first optical frequency converter 22, blocks the signal light 14, and includes the pump light 16 and the first frequency converted light 18. A first optical frequency selector 26 configured by an arrayed waveguide grating (AWG) and a multimode interference coupler 48 coupler, which emits two outgoing lights 24;

Further, the optical isolator device 11 receives the second outgoing light 24 emitted from the first optical frequency selector 26, mixes the excitation light 16 and the first frequency converted light 18 traveling in the same direction, A second frequency converted light 28 having a frequency f ₁ (wavelength 1.55 μm) is generated, and a third outgoing light 30 composed of the excitation light 16 and the first and second frequency converted lights 18 and 28 is emitted. The second optical frequency converter 32 made of a semiconductor optical amplifier 50 (nonlinear medium) is provided.

  Further, the optical isolator device 11 receives the third output light 30 emitted from the second optical frequency converter 32, emits the second frequency converted light 28, and the excitation light 16 and the first frequency converted light. A second optical frequency selector 34 including an arrayed waveguide diffraction grating 52 that blocks 18 is provided.

  In addition, the optical isolator device 11 includes a light incident port 36 through which the signal light 14 is incident and a light output port 38 through which the second frequency converted light is emitted.

  Furthermore, the optical isolator device 11 includes a plurality of optical waveguides 56, 59, 61, 63, 65, 67, 69, 71, 73 for optically connecting the semiconductor optical devices described above.

(2) Operation Next, the operation of the optical isolator device 11 according to the present embodiment will be described.

  First, the pumping light 16 having a wavelength of 1.57 μm and a light intensity of 20 mW is generated by the distributed feedback semiconductor laser 43. The generated excitation light 16 passes through the optical waveguide 59 and enters the light incident port on one side of the MMI coupler 54.

  On the other hand, the signal light 14 having a wavelength of 1.55 μm and a light intensity of 1 mW enters the light incident port 36 of the optical isolator device 11. The signal light 14 incident on the light incident port 36 passes through the optical waveguide 56 and enters the light incident port on the other side of the MMI coupler 54.

  The excitation light 16 and the signal light 14 are combined by the MMI coupler 54 and emitted to the optical waveguide 61. The excitation light 16 and the signal light 14 emitted to the optical waveguide 61 enter the first optical frequency converter 22.

The signal light 14 and the pumping light 16 incident on the first optical frequency converter 22 travel in the same direction in the first optical frequency converter 22. Then, the signal light 14 and the pumping light 16 are mixed (four-wave mixing) by the optical frequency converter 22 including the semiconductor amplifier 44 that functions as a nonlinear medium, and the third light having a _third frequency f ₃ (wavelength 1.59 μm). One frequency converted light 18 is generated. Here, the conversion efficiency from the signal light 14 to the first frequency converted light 18 is −5 dB.

  Next, the first emitted light 20 composed of the signal light 14, the excitation light 16, and the first frequency converted light 18 is emitted from the first optical frequency converter 22 to the optical waveguide 63.

  Next, the first outgoing light 20 emitted to the optical waveguide 63 enters the arrayed waveguide diffraction grating 46 that constitutes the first optical frequency selector 26. The excitation light 16 constituting the first emitted light 20 is emitted from the first light exit of the arrayed waveguide diffraction grating 46, passes through the optical waveguide 67, and forms the first optical frequency selector 26. The light enters the light entrance on one side of the coupler 48.

  The first frequency converted light 18 constituting the first emitted light 20 is emitted from the second light exit of the arrayed waveguide grating 46 and passes through the optical waveguide 65 to select the first optical frequency. Is incident on the light incident port on the other side of the MMI coupler 48 that constitutes the detector 26.

  On the other hand, the signal light 14 constituting the first emitted light 20 is emitted from a third light emitting port (not shown) of the arrayed waveguide diffraction grating 46. However, the third light exit of the arrayed waveguide grating 46 is not connected to the MMI coupler 48. Therefore, the signal light 14 is discarded in the InP substrate.

  The pumping light 16 and the first optical frequency converted light 18 that have entered the light incident ports on one side and the other side of the MMI coupler 48 are thus combined by the MMI coupler 48. Then, the second emitted light 24 composed of the excitation light 16 and the first frequency converted light 18 is emitted from the first optical frequency selector 26 to the optical waveguide 69.

  That is, the signal light 14 in the first outgoing light 20 emitted from the first optical frequency converter 22 is blocked by the first optical frequency selector 26, and the excitation light 16 and the first frequency converted light 18 are The second outgoing light 24 is emitted to the optical waveguide 69.

  Next, the second emitted light 24 emitted to the optical waveguide 69 is incident on the second optical frequency converter 32 including the semiconductor optical amplifier 50.

The excitation light 16 and the first frequency converted light 18 incident on the second optical frequency converter 32 travel in the same direction in the optical frequency converter 32. The pumping light 16 and the first frequency converted light 18 are mixed (four-wave mixing) by the second optical frequency converter 32 including the semiconductor amplifier 50 that functions as a nonlinear medium, and the first frequency f ₁ (wavelength). A second frequency converted light 28 having 1.55 μm) is generated. Here, the conversion efficiency from the first frequency converted light 18 to the second frequency converted light 28 is −5 dB.

  Next, the third emitted light 30 composed of the excitation light 16 and the first and second frequency converted lights 18 and 28 is emitted from the second optical frequency converter 32 to the optical waveguide 71.

  Next, the second emitted light 30 emitted to the optical waveguide 71 is incident on the second optical frequency selector 34 including the arrayed waveguide diffraction grating 52. The second frequency converted light 28 (wavelength 1.55 μm) constituting the third output light 30 is output from the first light output port of the arrayed waveguide diffraction grating 52, passes through the optical waveguide 73, and passes through the optical isolator. The light is emitted from the light exit port 38 of the device 11.

  On the other hand, the excitation light 16 (wavelength 1.57 μm) and the first frequency converted light 18 (wavelength 1.59 μm) constituting the third outgoing light 30 are the second and third (not shown) of the arrayed waveguide grating 52. It is emitted from the light exit. However, the second and third light exit ports of the arrayed waveguide grating 52 are not connected to the light exit port 38 of the optical isolator device 1. For this reason, the excitation light 16 and the first frequency converted light 18 are discarded in the InP substrate.

  That is, the third outgoing light 30 emitted from the second optical frequency converter 32 is incident on the second optical frequency selector 34. Then, the second frequency converted light 28 constituting the third emitted light 30 is emitted from the second optical frequency selector 34 and emitted from the light emitting port 38 of the optical isolator device 11. On the other hand, the excitation light 16 and the first frequency converted light 18 constituting the third outgoing light 30 are blocked by the second optical frequency selector 34.

  The second frequency converted light 28 emitted from the light emission port 38 is sent out into an optical device (not shown). Thereafter, the second frequency converted light 28 is reflected at a connection point of an optical member or the like existing on the propagation path, and returns to the light exit 38 as light 12 again.

The return light 12 that has entered the light exit port 38 passes through the optical waveguide 73 and enters the first light exit port of the arrayed waveguide grating 52 that constitutes the second optical frequency selector 34. The return light incident on the first light exit of the arrayed waveguide grating 52 follows the opposite path to the second frequency converted light 28 having the same frequency f ₁ in the arrayed waveguide grating 52, The light is emitted from the light incident port of the waveguide diffraction grating 52 to the optical waveguide 71. That is, the return light 12 passes through the second optical frequency selector 34. However, the transmittance of the arrayed waveguide diffraction grating 52 is not so high and is -13 dB.

  The return light 12 emitted to the optical waveguide 71 is incident on the second optical frequency converter 32 including the optical amplifier 50.

  The return light 12 incident from the optical waveguide 71 travels in the optical frequency converter 32 in the opposite direction to the excitation light 16 incident from the optical waveguide 69 connected to the side opposite to the optical waveguide 71. For this reason, the return light 12 is not mixed with the excitation light 16, passes through the optical frequency converter 32, and is emitted to the optical waveguide 69.

  The return light 12 emitted to the optical waveguide 69 enters the MMI coupler 48 that constitutes the first optical frequency selector 26. The return light 12 that has entered the MMI coupler 48 is emitted to the optical waveguide 65 and the optical waveguide 67.

  The return light 12 emitted to the optical waveguide 65 and the optical waveguide 67 is incident on the first and second light exit ports of the arrayed waveguide diffraction grating 46 constituting the first optical frequency selector 26. The return light 12 incident on the first and second light exit ports of the arrayed waveguide diffraction grating 46 travels in the arrayed waveguide diffraction grating toward the light entrance port connected to the waveguide 63.

  However, the frequency (wavelength 1.55 μm) of the return light 12 is the same as the frequency (wavelength 1.55 μm) of the signal light 14 emitted to the third light exit of the arrayed waveguide grating 46. Therefore, the return light 14 that has entered the first and second light exit ports does not reach the light entrance to which the optical waveguide 63 is connected (of the arrayed waveguide diffraction grating 46). That is, the return light 12 is blocked by the first optical frequency selector 26. At this time, the return light 12 is attenuated by -40 dB.

  For this reason, the return light 12 does not travel further in the optical isolator device 11. That is, the return light 12 does not reach the light incident port 36. In this way, operation as an optical isolation is realized. Note that the separation ratio of the optical isolator device 11 is −53 dB (= −13 dB−40 dB), as is apparent from the above description.

  The optical isolator device 11 in the present embodiment is composed of a semiconductor optical device that can be formed on a semiconductor substrate. Therefore, the optical isolator device 11 in this embodiment can be integrated on a semiconductor substrate together with other semiconductor optical devices.

(Embodiment 2)
The present embodiment relates to an integrated semiconductor optical device 58 in which the above-described optical isolator device 11 and another semiconductor optical device are integrated.

(1) Device Configuration FIG. 3 is a perspective view of an integrated semiconductor optical device 58 in the present embodiment.

  The integrated semiconductor optical device 58 includes the optical isolator device 11 described in the first embodiment and the semiconductor mode-locked laser 60 formed on the semiconductor substrate (n-type InP substrate 42) on which the optical isolator device 11 is formed. It is configured. Further, the light exit port 36 of the semiconductor mode-locked laser 60 is optically connected to the light entrance port of the optical isolator device 11 via the optical waveguide 56.

  The configuration of the optical isolator device 11 is the same as that shown in the first embodiment. Here, the active layers of the distributed feedback semiconductor laser 43 and the semiconductor optical amplifiers 44 and 50 are composed of InGaAs / InGaAs multi-quantity well active layers. Further, the MMI 48, 54, the AWG (arrayed waveguide diffraction grating) 46, 52, and the optical waveguides 56, 59, 61, 63, 65, 67, 69, 71, 73 are composed of an InGaAs core layer and an i-InP cladding layer. It is configured. The length of the semiconductor optical amplifiers 44 and 50 is 1 mm.

  On the other hand, the semiconductor mode-locked laser 60 is composed of a gain unit 62 and a saturable absorber unit 64. The active layer of the gain unit 62 and the light absorption layer of the saturable absorption unit 64 are composed of the same InGaAs / InGaAs multi-quantum well active layer as the active layers of the distributed feedback semiconductor laser 43 and the semiconductor optical amplifiers 44 and 50.

  The optical isolator device 11 and the semiconductor mode-locked laser 60 are embedded with high resistance InP 66 doped with iron (Fe).

(2) Usage Method Next, a usage method of the integrated semiconductor optical device 58 in the present embodiment will be described.

  The integrated semiconductor optical device 58 can be directly connected to, for example, a high-speed optical time division multiplexing (OTDM) optical communication system without using a conventional optical isolator (FIG. 8), and can be used as a light source.

  A high-speed optical time division multiplexing (OTDM) optical communication system uses an optical pulse train generated by an optical pulse light source such as a semiconductor mode-locked laser 60 as signal light. However, when the optical pulse train emitted from the semiconductor mode-locked laser 60 is directly incident on the OTDM optical communication system, jitter occurs in the optical pulse train emitted from the semiconductor mode-locked laser 60 due to the return light generated in the system.

  The integrated semiconductor optical device 58 in the present embodiment emits the optical pulse train generated by the semiconductor mode-locked laser 60 via the optical isolator device 11 integrated on the same substrate. Accordingly, the return light is blocked by the optical isolator device 11 and does not enter the semiconductor mode-locked laser 60. For this reason, jitter does not occur in the optical pulse train generated by the semiconductor mode-locked laser 60.

  That is, in the integrated semiconductor optical device 58 in the present embodiment, since the semiconductor mode-locked laser 60 and the optical isolator device 11 are integrated, jitter occurs in the pulse train even when directly connected to the OTDM optical communication system. There is nothing.

(3) Manufacturing Method Finally, a method for manufacturing the integrated semiconductor optical device 58 shown in FIG. 3 will be described according to the sectional process diagrams shown in FIGS.
Here, FIG. 4 and FIG. 5 schematically show the cross section taken along the line AA ′ shown in FIG. 3 from the direction of the arrow. 6 and 7 schematically show a cross section taken along the line BB ′ shown in FIG. 3 when viewed from the direction of the arrow.

  First, as shown in FIG. 4A and FIG. 6A, an n-type InP clad layer 68, an InGaAsP / P on the n-type InP substrate 42 using a metal organic chemical vapor deposition method (MO-CVD method). An active layer 70 made of an InGaAs multiple quantum well (λg = 1.55 μm), a p-type InP clad layer 72, and a p-type contact layer 74 are sequentially grown flat (first step).

  These growth layers are processed into the mode-locked laser 60, the distributed feedback semiconductor laser 43, and the semiconductor optical amplifiers 44 and 50 by the following steps. Note that λg is a value obtained by converting the band gap into a wavelength.

  Next, as shown in FIG. 4B and FIG. 6B, using the Inductively Coupled Plasma (ICP) technique and the photolithography technique, the MMI 48, 54, the AWGs 46, 52, and the optical waveguide 56 are used. , 59, 61, 63, 65, 67, 69, 71, 73 are etched to remove the layer grown in the first step (second step).

  Next, as shown in FIG. 4C and FIG. 6C, the i-InP lower cladding layer 76, the InGaAsP / InGaAs multiple quantum well are formed on the portion removed by the second step by using MO-CVD. A core layer 78 and an i-InP upper cladding layer 80 made of (λg = 1.35 μm) are grown (third step).

  Next, as shown in FIG. 5A and FIG. 7A, using the ICP technique and the photolithography technique, the mode-locked laser 60, the distributed feedback semiconductor laser 43, the semiconductor optical amplifiers 44 and 50, and the MMI 48 and 54 are used. The layers grown in the first and second steps are removed by etching except the portions that become the AWGs 46 and 52 and the optical waveguides 59, 61, 63, 65, 66, 69, 71, and 73 (fourth). Process).

  Next, as shown in FIG. 5B and FIG. 7B, Fe-doped high-resistance InP 66 is grown on the portion removed in the fourth step by MO-CVD (fifth). Process).

  Finally, as shown in FIG. 5C and FIG. 7C, the p-type contact layer 74 in the portions to be the mode-locked laser 60, the distributed feedback semiconductor laser 43, and the semiconductor optical amplifiers 44 and 50 is p-type. An electrode 82 (Ti / Pt / Au) is formed. Further, an n-type electrode 84 (AuGe / Au) is formed on the back surface of the n-type InP substrate.

  As described above, in the present embodiment, the first and second optical frequency converters 22 and 32 perform frequency conversion using four-wave mixing based on the third-order nonlinear effect. However, the phenomenon that can be used for frequency conversion is not limited to four-wave mixing. For example, the optical frequency converters 22 and 32 may use difference frequency generation by a second-order nonlinear effect.

Further, the nonlinear medium constituting the first and second frequency converters is not limited to a gain medium such as an SOA active layer. It may be a passive optical waveguide having In _x Ga _1-x As _y P _1-y as a core layer.

  The present invention can be used in the semiconductor optical device manufacturing industry, particularly in the semiconductor optical device manufacturing industry.

It is a conceptual diagram of the optical isolator apparatus which concerns on embodiment. FIG. 3 is a schematic diagram showing a planar structure of the optical isolator device in the first embodiment. It is a perspective view of the integrated semiconductor optical device in Embodiment 2. FIG. FIG. 16 is a first cross-sectional view showing a manufacturing process of the integrated semiconductor optical device in the second embodiment. FIG. 22 is a second diagram illustrating a manufacturing process of the integrated semiconductor optical device according to the second embodiment in cross section. FIG. 11 is a third diagram illustrating a manufacturing process of the integrated semiconductor optical device according to the second embodiment in cross section. FIG. 14 is a fourth diagram illustrating a manufacturing process of the integrated semiconductor optical device according to the second embodiment in cross section. It is a figure explaining the structure of the conventional isolator apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Conventional optical isolator apparatus 2 1st polarizer 3 Transmission axis 4 of 1st polarizer 2nd polarizer 5 Transmission axis 6 of 2nd polarizer Magnetic field 8 Incident light 9 Polarization direction 10 of incident light Faraday rotation Element 11 Optical isolator device 12 Return light 13 Polarization direction 14 of return light Signal light 16 Excitation light 18 First frequency conversion light 20, 24, 30 Emission light 22 First optical frequency converter 26 First optical frequency selector 28 Second frequency converted light 32 Second optical frequency converter 34 Second optical frequency selector 36 Light entrance port 38 Light exit port 42 n-type InP substrate 43 Distributed feedback semiconductor laser 44, 50 Semiconductor optical amplifier 46, 52 Arrayed waveguide diffraction grating 48, 54 Multimode interference coupler 56, 59, 61, 63, 65, 67, 69, 71, 73, 80 Optical waveguide 58 Integrated semiconductor optical device 60 Semiconductor mode lockless User 62 Gain section 64 Saturable absorption section 66 High resistance InP doped with iron (Fe)
68 n-type InP cladding layer 70 active layer 72 p-type InP cladding layer 74 p-type contact layer 76 i-InP lower cladding 78 core layer 80 i-InP upper cladding layer

Claims (5)

The signal light having the first frequency and the pumping light having the second frequency traveling in the same direction as the signal light are mixed to generate the first frequency converted light having the third frequency, and the signal light A first optical frequency converter made of a nonlinear medium that emits first emitted light made of the excitation light and the first frequency converted light;
The first light emitted from the first optical frequency converter is incident, blocks the signal light, and emits the second emitted light composed of the excitation light and the first frequency converted light. A frequency selector;
The second outgoing light emitted from the first optical frequency selector is incident, the pumping light traveling in the same direction and the first frequency converted light are mixed, and the second frequency converted light having the first frequency is mixed. A second optical frequency converter made of a nonlinear medium that emits a third outgoing light consisting of the excitation light and the first and second frequency converted lights,
A second optical frequency selector that receives the third emitted light emitted from the second optical frequency converter, emits the second frequency converted light, and blocks the excitation light and the first frequency converted light; ,
A light entrance through which the signal light is incident;
Comprising a light exit through which the second frequency converted light is emitted;
A second optical frequency selector transmits the return light reflected by the second frequency converted light and returned to the light exit;
A second optical frequency converter transmits the return light transmitted by the second optical frequency selector without being mixed with the excitation light;
An optical isolator device in which a first optical frequency selector blocks the return light transmitted by a second optical frequency converter.   2. The optical isolator device according to claim 1, wherein the first and second optical frequency converters are semiconductor optical amplifiers.   The first and second optical frequency converters, the first and second optical frequency selectors, the light entrance and the light exit are formed on the same semiconductor substrate. 3. The optical isolator device according to 2.   4. The optical isolator device according to claim 1, wherein the substrate is a substrate made of indium phosphorus. An optical isolator device according to claim 3 or 4,
An integrated semiconductor optical device comprising a semiconductor optical device integrated on the semiconductor substrate and having a light exit opening optically connected to the light entrance opening.

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