JP2760290B2 - Optical module optical axis adjustment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical axis adjusting method,
In particular, the present invention relates to a method for adjusting an optical axis of an optical module for coupling outgoing light of a semiconductor laser to an optical fiber.

[0002]

2. Description of the Related Art As a conventional optical axis adjustment method of an optical module, for example, there is a hill-climbing method.

A conventional optical module optical axis adjusting method using the hill climbing method will be described with reference to the drawings.

FIG. 8 is an explanatory view showing a conventional hill-climbing method.

The optical fiber _is moved relative _{to the} semiconductor laser unit at a predetermined feed pitch 21 in one axial direction, and the transmitted light output of the optical fiber is compared at each of the moved positions to search for a position where a peak 22 can be obtained. This procedure is repeated independently for the three axes X, Y, and Z as long as the light output increases. Usually, the feed pitch is set to several stages in order to shorten the search time, and the adjustment is performed in the order of coarse adjustment and fine adjustment.

FIG. 9 is an explanatory diagram showing a case where the light output of the conventional example has a local peak 23 instead of a monotonically increasing or monotonically decreasing distribution.

If such a local peak 23 is present, the search ends at the local peak 23, so that the true peak 24 cannot be reached by the conventional hill-climbing method.

As an example using vector search other than the hill-climbing method, there is one disclosed in Japanese Patent Application Laid-Open No. 62-75508.

Next, the structure disclosed in Japanese Patent Application Laid-Open No. 62-75508 will be described with reference to the drawings.

FIGS. 10 and 11 are a sectional view and a perspective view, respectively, showing the structure of a laser diode for explaining the principle of optical axis adjustment in a conventional example.

The laser diode chip 51 is a submount.
52 is fixed, the sub-mount 5 2 is fixed to the frame 53. On the other hand, the optical fiber 54 is fixed so as to penetrate a hole formed in the center of a column 55 having one fixed to the frame 53. The alignment of the optical axis between the chip 51 and the optical fiber 54 is performed so that the output from the optical fiber is maximized by plastically deforming the column 55 in the X, Y, and θ directions.

Next, the optical axis positioning method will be described in detail.

The light output at the initial positions X ₀ , Y ₀ , θ ₀ is f _0, and the gradient of the light output surface in the X-axis direction is obtained from the difference between the light output before and after the movement of △ X ₁ from equation (51).

[0014]

Here, f _X1 is the optical output after the movement of ΔX ₁ .

Next, the image is moved by ΔY ₁ , and the gradient in the Y-axis direction is similarly obtained from equation (52).

[0017]

Further, it is moved by △ θ ₁ and the gradient in the θ direction is obtained from equation (53).

[0019]

Since the light output surface has been moved in the X, Y, and θ directions, the slope s and the height h of the light output surface are determined as follows.

[0021]

H ₁ = fθ ₁ (55) The next (second) movement amounts △ X ₂ , 移動 Y ₂ , △ θ ₂ in the X, Y, and θ directions using (51) to (55) are

[0023]

Is determined. Here, G = As ₁ ^m / h ₁ ⁿ (59) A, m, and n are constants, and the optimum value is obtained by experiment. Similarly, the amount of movement in the X, Y, and θ directions for the Nth time △
X _n , △ Y _n , △ θ _n are

[0025]

It is determined that Here, G = As _n-1 ^m / h _n-1 ⁿ (63)

[0027]

H _n-1 = fθ _n-1 (65) Hereinafter, the movement is continued in this manner, and the movement is performed when N exceeds a certain number of times, s falls below a certain value, or h exceeds a certain value. To end.

This conventional example applies a gradient measurement to a search,
This is a vector search. The number of movements can be reduced as compared with the conventional hill-climbing method.

[0030]

Among the above-mentioned conventional optical module optical axis adjustment methods, the general hill-climbing method requires time for searching because peaks are sequentially searched at a fixed pitch for each axis independently. There is a problem that a true peak cannot be reached if a local peak exists.

Further, a method using the conventional gradient measurement is as follows.
Although it is possible to efficiently reach the vicinity of the peak vector-wise, there is no means for confirming the peak, and there is a problem that it may remain at a local peak like the hill-climbing method.

[0032]

Optical module optical axis adjusting method of the present invention According to an aspect of, the optical axes of the semiconductor laser unit and an optical fiber having a lens for condensing light emitted from the semiconductor laser and the semiconductor laser Light transmitted from optical fiber
In order to maximize the output , the relative positions of the two are set in X, Y and Z directions in the X and Y directions perpendicular to the optical axis and the Z direction in the optical axis direction.
In the optical axis adjustment method of an optical module for performing alignment in a three-dimensional space using the three-axis adjustment stage, the spatial power characteristic of optical coupling in a plane perpendicular to the optical axis approximates a Gaussian laser power distribution. Assuming that it is possible, it is arranged at 90 degrees on the circumference of a cross section perpendicular to the optical axis.
4 points and a total of 5 points of light output and half of the measurement circle
Using the diameter, the collection of the laser in the X direction at the optical axis position
A light beam radius w _X and Y directions of the focused beam radius w _Y determined
Because the average value of w _X and the w _Y and the radius of the measuring circle
The first feature is that the amount of positional deviation from the center of the optical axis in two directions orthogonal to is calculated and alignment is performed.

In the optical module optical axis adjusting method according to the present invention, the spatial power characteristic of the optical coupling in the optical axis direction is TEM ₀₀
Based on the assumption that the wavefront spread of the Gaussian beam in the mode can be approximated , the beam diameter obtained at the optical axis position and the standard size
The distance to the optical axis focal position is calculated using the beam diameter at the optical axis focal position of the sample, and the next search position is measured.
It is set between the position and the calculated optical axis focal position, and
A second feature is to search for a position where the maximum light output can be obtained while correcting the constant optical axis focal position .

[0034]

The principle of the present invention will be described below before describing the embodiments of the present invention _.

The laser beam emitted from the LD and condensed by the lens has a Gaussian beam radiation distribution represented by the equation (10).

I (r) = (2P / πw ² ) EXP (−2r ² / w ² ) (10) where I (r): light intensity at a distance r from the center P: total power of the beam w: The beam radius at which the output intensity becomes 1 / e ² (13.5%) e: exponential function r: distance from the center The optical system of the Gaussian beam is described in July 1993 by Hideaki Yamada, K. It is described on page 3 of “Laser & Optics Guide No. 3 (2), Laser Accessories and Detectors” published by No Melles Griot Co., Ltd.

FIG. 3 is a graph showing the transmitted light output of the optical fiber measured by shifting the position of the optical fiber from the center of the optical axis on a section perpendicular to the optical axis after adjusting the optical axis in the sample of the optical module shown in FIG. It is.

The mark + indicates the measurement result in the X direction and the _{mark Δ} indicates the measurement result in the Y axis direction, which is normalized and displayed with the maximum power at the center of the optical axis.
The solid line is the intensity distribution of the Gaussian beam obtained by calculating the beam radius w at 13.5% of the peak output from the measurement result and substituting it into the equation (10). Similarly, it is normalized and displayed with the maximum power at the center. Both are in good agreement, indicating that the spatial power characteristic of the optical coupling of the optical module can be approximated to a Gaussian laser power distribution.

Next, a method for obtaining the beam radius and the amount of displacement from the center of the optical axis at each point in the direction of the optical axis from the light output at five points in the cross section perpendicular to the optical axis of the Gaussian beam will be described.

FIG. 4 is a plan view showing the definition of measurement points in a section perpendicular to the optical axis.

Four points P _{X +} , P _X− , P _{Y +} , and P _Y− are _equally arranged at 90 ° on the circumference of the circle with respect to the measurement center P ₀ .

It should be noted that the X and Y axes in FIG.
It coincides with the feed axis directions of the X and Y axes of the stage 7. The light output (I ₀ , I _{X +} , I _X− , I
_{Y +} , I _Y− ) are given by equations (11) to (15).

I ₀ = K · EXP [−2 (△ X ₂ + △ Y ₂ ) / w ₂ ] (11) I _{X +} = K · EXP [−2 ｛(△ X + R) ² + {Y ² } / w ² ] (12) I _X− = K · EXP [−2 ｛(△ X−R) ² + {Y ² } / w ² ] (13) I _{Y +} = K · EXP [−2 ｛△ X ² + ( ΔY + R) ² ｝ / w ² ] (14) I _Y− = K · EXP [−2 ｛△ X ² + (△ Y−R) ² ｝ / w ² ] (15) where, w: beam radius K : 2P / πw ² R: circle having a radius of △ X, △ Y: positional deviation amount formula from the optical axis (11) ln than to formula _{(13) (I X + /} I 0) = - 2 (R 2 + 2RX) / w ² (16) ln (I _X− / I ₀ ) = − 2 (R ² −2RX) / w ² (17) When Equations (16) and (17) are solved for w, w = 2R / [− _{{ln (I X + / I} 0) + ln (I X- / I 0)}] 1/2 (18) in the same manner, w = 2R / [- { ln (I Y + / I 0) + ln ( _Y- / I _0)}] is ^half (19).

In the actual measurement, since the beam diameters in the X-axis and Y-axis directions are different, the beam radius w _{X in the} X-axis direction is different.
From equation (18) _{a, w X = 2R / [-} {ln (I X + / I 0) + ln (I X- / I 0)}] 1/2 (20) In addition, the Y-axis direction beam radius w _Y From equation (19), it is assumed that w _Y = 2R / [− {ln (I _{Y +} / I ₀ ) + ln (I _Y− / I ₀ )}] ^1/2 (21), and the beam radius w is w = ( given by _{w X + w Y) / 2} (22).

Using the obtained w, the amount of axis deviation {X, △
How to determine Y will be described. From the equations (12) and (13), ln ( _IX− / IY ₊ ) = 8R △ X / w ² (23) When this is solved for ΔX, ΔX = w ² × ln (I _X− / I _{Y +} ) / (8R) (24) Similarly, ΔY is also given as ΔY = w ² × ln (I _X− / I _{Y +} ) / (8R) (25)

Next, the spread of the laser beam in the optical axis direction will be described.

The wavefront spread (w) of the Gaussian beam in the TEM00 mode is given by equation (26).

W (z) = w ₀ ｛1+ (λz / πw ₀ ² ) ² ｝ ^1/2 (26) w (z): Beam radius at propagation distance r from the beam waist w ₀ : Beam waist radius λ: Wavelength z of laser beam: Propagation distance from beam waist TEM is an abbreviation of "transverse electronomagetic". A specific TEM wave of an electromagnetic TEM wave in which both the electric field vector and the magnetic field vector are always perpendicular to the propagation direction is guided. A mode that propagates through tubes and cavities.

FIG. 5 shows the result of measuring the optical output of the sample of the optical module shown in FIG. 1 by moving the optical fiber in the optical axis direction from the position where the maximum optical coupling is obtained.

The theoretical values are the beam waist radius at the position where the maximum optical coupling is obtained and the wavelength of the laser beam (1.3 μm).
m) into Equation (26) to determine the beam radius w at each optical axis position z, and substitute the obtained beam radius w into Equation (10) to obtain the maximum light output (zero optical axis deviation) at each optical axis position. It is what was asked. Although the deviation from the theoretical value tends to be large in the vicinity of the beam waist, it is understood that the deviation substantially coincides with the theoretical value.

Accordingly, the propagation distance from the beam waist, that is, the optical axis focal position, is obtained from the equation (27) obtained by solving the equation (26) for z using a substantially constant known beam waist radius and the beam diameter at each optical axis position. Can be obtained.

Z = (πw ₀ ² / λ) ｛{(w / w ₀ ) ² -1} ^1/2 (27)

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view showing an example of an optical module to which the present invention is applied, and FIG. 2 is a view for explaining how the optical axis of the optical module of FIG. 1 is adjusted on an optical axis adjustment stage. FIG.

In FIG. 1, this optical module is an LD
The element package 1, the lens holder 3 fixed to the LD element package 1, the condenser lens 2 incorporated in the lens holder 3, the pipe 5 with the flange, and the lens holder 3 through the pipe 5 with the flange And an optical fiber terminal 4 having a fixed optical fiber 4a.

Next, a method of adjusting the optical axis of the optical module on the optical axis adjusting stage will be described with reference to FIG.

The LD unit 6 including the LD element package 1, the condenser lens 2 and the lens holder 3 shown in FIG.
The LD chuck 9a on the XY stage 7 composed of the X stage 7a and the Y stage 7b, and the optical fiber 4a of the optical fiber terminal 4 is attached to the fiber chuck 9 on the Z stage 8.
b. The LD unit 6 is moved on the section perpendicular to the optical axis by the XY stage 7 and the optical fiber 4a is moved in the optical axis direction by the Z stage 8 to transmit the transmitted light from the optical fiber 4a in a three-dimensional space. The two are aligned so that the output is maximized.

Next, an optical axis adjusting method according to an embodiment of the present invention will be described with reference to FIG.

FIG. 6 is a flowchart showing an optical axis adjustment method on the premise that the spatial power characteristic of optical coupling can be approximated to a Gaussian beam.

[0059] First, in step S1, to calculate the predicted beam radius w _e at the current position of the optical axis with a beam waist radius distance z and the standard samples from the current position of the optical axis to the optical axis focal point of the standard sample , The radius w _e /
The light output at four points and the center on the circumference of 4 is measured. In step S3, the beam radius, the amount of axial deviation, and the optical axis focal position are obtained using the optical output measured in step S2, and in step S4,
The position is corrected based on the amount of axis deviation obtained in step S3, and step S5
Then, the peak is determined from the beam radius and the distance to the optical axis focal position obtained in step S3. If the peak condition is satisfied, the subsequent search is stopped. In step S6, if the peak condition is not satisfied in step S5, the next search optical axis position z is specified, and the process returns to step S1.

In the peak judgment in the step S5, for example, the distance to the optical axis focal position z ≦ 20 μm....... (A) 0.95 ≦ (measured beam radius / (Experimentally determined beam waist radius) ≦ 1.05 (b) If either one is satisfied, it can be determined that the peak is reached.

In the step S6 for designating the next search position in the direction of the optical axis, the next search position is set, for example, at the center between the current position and the focus position of the optical axis, or at a position where the beam diameter becomes half. The exponential function converges to the optical axis focal position, and the peak position can be reliably searched by correcting the predicted optical axis focal position based on the successive calculation results.

FIG. 7 shows an example of the result of a search performed on actual samples based on the optical axis adjustment flow of FIG.

The optical axis was adjusted by the hill-climbing method, and a search was performed based on the flow of FIG. FIG. 7 shows changes in the number of searches and the normalized power by normalizing the power with the known maximum optical power. The radius of the measurement circle at each position is 1/4 of the predicted beam radius
The next search position in the optical axis direction was set by calculating a position where the beam radius becomes half. When the search position approaches the optical axis focal position and the beam diameter reaches less than twice the beam waist diameter, the next search position is set at the center between the predicted optical axis focal position and the current position.

The search was repeated according to the above rules, and a peak was determined when the error between the beam diameter and the beam waist diameter was within 5% or the distance to the optical axis focal position was 30 μm or less.

As a result, it can be seen from FIG. 7 that the search has almost reached the peak in six searches.

After the peak is determined in step S5 in FIG. 6, the search position is changed in the optical axis direction by, for example, 10 μm steps.
Steps S2 to S4 are performed, and by sequentially comparing the light outputs, the position where the light output becomes maximum can be accurately searched.

[0067]

As described above, according to the present invention, an optical module optical axis adjusting method of the present invention, the spatial power characteristics of optical coupling with a can be approximated to the Gaussian beam, 5 points in _a cross section perpendicular _{to the} arbitrary optical axis In addition to calculating the beam radius and the amount of axial deviation from the optical output and correcting the amount of positional deviation, the distance between the measured beam radius and the beam waist radius at the optical axis focal position of the experimentally determined standard sample to the optical axis focal position is calculated. It can be estimated, and it is efficient and reliable without being affected by local peaks or deviations from approximations while sequentially setting and correcting the next search position to approach the optical axis focus position with reliable prediction There is an effect that the optical axis can be aligned.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an optical module for explaining an embodiment of an optical axis adjusting method according to the present invention.

FIG. 2 is a diagram for explaining how an optical axis of the optical module of FIG. 1 is adjusted on an optical axis adjustment stage.

FIG. 3 is a graph illustrating an intensity distribution in a cross section perpendicular to the optical axis of optical coupling in the optical axis adjustment method according to the present embodiment.

FIG. 4 is a plan view showing the definition of measurement points for explaining the principle of the optical axis adjustment method of the present embodiment.

FIG. 5 is a graph illustrating the intensity distribution in the optical axis direction of optical coupling in the optical axis adjustment method according to the present embodiment.

FIG. 6 is a flowchart of an optical axis adjustment method according to the present embodiment.

FIG. 7 is a graph illustrating a search example of the optical axis adjustment method according to the present embodiment.

FIG. 8 is an explanatory diagram for explaining a conventional hill-climbing method.

FIG. 9 is an explanatory diagram for explaining a problem of the conventional hill-climbing method.

FIG. 10 is a cross-sectional view showing a configuration of an optical module for explaining another optical axis adjusting method of the conventional example.

11 is a perspective view of the optical module shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 LD element package 2 Condensing lens 3 Lens holder 4 Optical fiber terminal 4a Optical fiber 5 Pipe with flange 6 LD unit 7 XY stage 7a X stage 7b Y stage 8 Z stage 9a LD chuck 9b Fiber chuck

Claims (5)

(57) [Claims] An optical axis of an optical fiber and a semiconductor laser unit having a semiconductor laser and a lens for condensing light emitted from the semiconductor laser , the transmitted light from the optical fiber.
In order to maximize the output , the relative positions of the two are set in X, Y and Z directions in the X and Y directions perpendicular to the optical axis and the Z direction in the optical axis direction.
In the optical axis adjustment method of an optical module for performing alignment in a three-dimensional space using the three-axis adjustment stage, the spatial power characteristic of optical coupling in a plane perpendicular to the optical axis approximates a Gaussian laser power distribution. Assuming that it is possible, it is arranged at 90 degrees on the circumference of a cross section perpendicular to the optical axis.
4 points and a total of 5 points of light output and half of the measurement circle
Using the diameter, the collection of the laser in the X direction at the optical axis position
A light beam radius w _X and Y directions of the focused beam radius w _Y determined
Because the average value of w _X and the w _Y and the radius of the measuring circle
An optical module optical axis adjusting method, comprising calculating a positional deviation amount from a center of an optical axis in two directions orthogonal to each other and performing alignment. 2. On the assumption that the spatial power characteristic of optical coupling in the optical axis direction can be approximated to the wavefront spread of a Gaussian beam in a TEM ₀₀ mode, the optical power is obtained at the optical axis position.
Beam diameter and beam at the optical axis focus position of the standard sample
Calculating a distance to the optical axis focal position by using the diameter, the following probe
Between the measurement position and the determined optical axis focal position.
To the maximum light while correcting the sequentially estimated optical axis focal position.
2. The optical module optical axis adjusting method according to claim 1, wherein a position where an output is obtained is searched. 3. A light output (I _{X +} , I _X− , I _{Y +} , I _Y− , 4 points on the circumference of a section perpendicular to the optical axis and a total of 5 points at the center.
Using I ₀ ) and the radius (R) of the measurement circle, w _X = 2R / [− {ln (I _{X +} / I ₀ ) + ln (I _X− / I ₀ )}] ^1/2 and w _Y = 2R / [− {ln (I _{Y +} / I ₀ ) + ln (I _Y− / I ₀ )}] ^1/2, and the laser collection at the optical axis position represented by w = (w _X + w _Y ) / 2 Light beam radius w
, And using the obtained beam radius w, ΔX = w × ln ( _IX− / _{IX +} ) / (8R) and ΔY = w × ln ( _IY− / _{IY +} ) / (8R) Position shift amount Δ from the optical axis center in two orthogonal directions
3. The optical module optical axis adjusting method according to claim 1, wherein X and .DELTA.Y are obtained and the positional deviation amount is corrected. 4. The optical module light according to claim 3, wherein two orthogonal directions in a cross section perpendicular to the optical axis coincide with feed directions in the X and Y directions of the three-axis adjustment stage. Axis adjustment method. 5. The method according to claim 5, wherein the radius of the measurement circle at the search position is
1/4 to 2 times the predicted beam radius at the search position
3. The method according to claim 2, wherein the degree is set to a degree.
3. The method for adjusting the optical axis of the optical module according to 3 .