Semiconductor laser device

Abstract

Provided is a semiconductor laser device having an internal monitor-photodiode design and having improved long-term output stability.　A laser diode (LD) chip 110 is an end-face emission type and emits forward laser output L1 from a forward end face S1. A glass window 150 is provided on the forward end-face S1 side of the LD chip 110 and reflects a portion of the forward laser output L1. A light-receiving element 170 is provided on the rearward end-face S2 side of the LD chip 110 so that the light-receiving element can receive reflected forward laser output L4, which is forward laser output L3 that is reflected by the glass window 150. Power Pr' of rearward laser output L6, emitted from the rearward end face S2 of the LD chip 110 and incident on the light-receiving element 170, is 1 / 3 or less of power Pf" of the reflected forward laser output L4 incident on the light-receiving element 170.

Description

semiconductor laser device

[0001] The present disclosure relates to a semiconductor laser device.

[0002] Some semiconductor laser devices house an edge-emitting semiconductor laser (LD: Laser Diode) chip together with a photodiode (PD: Photo Diode) in a package such as a CAN package. The internal PD built into the package is located on the rear end face of the LD chip and receives the rear optical output leaking from the rear end face of the LD chip. Because the power of the rear optical output correlates with the power of light emitted from the front end face of the LD chip (forward optical output), the output of the internal PD indirectly indicates the power of the forward optical output of the LD chip.

[0003] The output of the internal PD is taken out of the package. A drive circuit outside the package feedback controls the drive current supplied to the LD chip (APC: Automatic Power Control) to keep the output of the internal PD constant. This stabilizes the forward optical output of the LD chip.

[0004] Patent No. 5319397

[0005] The present inventors have studied LDs with internal PD monitors and have come to recognize the following problems.

[0006] The internal PD monitor method indirectly monitors the forward optical output Pf by utilizing the backward optical output Pr. In this method, the ratio Pr / Pf of Pr to Pf is required to be constant regardless of temperature or wavelength. Let Rf be the reflectance of the front end face of the LD chip, and Rr be the reflectance of the rear end face. The power of light emitted from the front end face Pf and the power of light emitted from the rear end face Pr each satisfy the following relationships: Pf ∝ 1 / √Rf × (1-Rf) Pr ∝ 1 / √Rr × (1-Rr)

[0007] Therefore, the ratio of Pr to Pf, Pr / Pf, is expressed by the following equation (1): Pr / Pf=√Rf / √Rr×(1−Rr) / (1−Rf) (1)

[0008] The facets of LDs operating in the GaN / InGaN wavelength band lack optical stability in the dielectric reflective film that determines their reflectivity, the interface between the dielectric reflective film and the semiconductor, and the state of the semiconductor in the vicinity. This optical degradation of the facet film structure is unavoidable over long-term operation. As a result of this optical degradation, the reflectivities Rf and Rr fluctuate. If Pr / Pf deviates from the designed wavelength dependency (PD sensitivity compensation slope characteristic as in Patent Document 1), the external optical output Po deviates from the tolerance range of the specified APC specifications.

[0009] This problem can occur even under operating conditions where the environmental temperature is kept constant and the oscillation wavelength is maintained constant, and is common to internal PD monitoring methods in which the majority of the light received by the PD comes from the backward optical output from the LD, regardless of whether the film structure has an appropriately controlled wavelength dependence of Pr / Pf.

[0010] The present disclosure has been made in view of the above-mentioned problems, and one exemplary purpose of an embodiment thereof is to provide a semiconductor laser device with an internal PD monitor system that has improved long-term output stability.

[0011] One aspect of the present disclosure relates to a semiconductor laser device. The semiconductor laser device includes an edge-emitting semiconductor laser chip that emits forward optical output from a front end facet, a glass window provided on the front end face of the semiconductor laser chip and that reflects a portion of the forward optical output, and a light-receiving element provided on the rear end face of the semiconductor laser chip so as to receive the reflected forward optical output that is the forward optical output reflected by the glass window. The power of the backward optical output radiated from the rear end face of the semiconductor laser chip that is incident on the light-receiving element is one-third or less of the power of the reflected forward optical output that is incident on the light-receiving element.

[0012] Any combination of the above components, or mutual substitution of the components or expressions of the present disclosure between methods, devices, systems, etc., are also valid aspects of the present disclosure.

[0013] According to an aspect of the present disclosure, it is possible to improve the long-term output stability of a semiconductor laser device using an internal PD monitor.

[0014] 13 is a cross-sectional view of a semiconductor laser device according to a first embodiment. FIG. 14 is a diagram explaining the operation of the semiconductor laser device of FIG. 1. FIG. 15 is a cross-sectional view of a semiconductor laser device according to a second embodiment. FIG. 16 is a plan view of the semiconductor laser device of FIG. 3. FIG. 17 is a cross-sectional view of a semiconductor laser device according to a third embodiment. FIG. 18 is a cross-sectional view of a semiconductor laser device simplified for design purposes. FIG. 19 is a diagram showing the powers Pf" and Pr' of light incident on the light receiving element when Rwin=5%. FIG. 20 is a diagram showing the powers Pf" and Pr' of light incident on the light receiving element when Rwin=3%. FIG. 21 is a diagram showing the powers Pf" and Pr' of light incident on the light receiving element when Rwin=2%. FIG. 22 is a cross-sectional view of a glass window. FIG. 23 is a cross-sectional view of an LD chip according to an example. FIG. 24 is a diagram showing the reflectance and transmittance of the rear end face of the LD chip of FIG. 12. FIG. 25 is a cross-sectional view of an LD chip according to an example. FIG. 26 is a diagram showing the transmittance of the rear end face of the LD chip of FIG. 14.

[0015] (Summary of the Embodiments) A summary of some exemplary embodiments of the present disclosure will be provided. This summary is intended as a prelude to the detailed description that follows, or to provide a basic understanding of the embodiments. The summary is intended to briefly explain some concepts of one or more embodiments and is not intended to limit the scope of the invention or disclosure. Furthermore, this summary is not an exhaustive overview of all possible embodiments, nor does it limit essential elements of the embodiments. For convenience, the term "one embodiment" may refer to one embodiment (example or variant) or multiple embodiments (examples or variants) disclosed herein.

[0016] A semiconductor laser device according to one embodiment includes an edge-emitting semiconductor laser chip that emits a forward optical output from a front end facet, a glass window provided on the front end face side of the semiconductor laser chip and that reflects a portion of the forward optical output, and a light-receiving element provided on the rear end face side of the semiconductor laser chip so as to receive the reflected forward optical output that is the forward optical output reflected by the glass window, wherein the power of the backward optical output radiated from the rear end face of the semiconductor laser chip that is incident on the light-receiving element is one-third or less of the power of the reflected forward optical output that is incident on the light-receiving element.

[0017] The light incident on the light-receiving element may include the backward optical output emitted from the rear end facet of the semiconductor laser chip and the reflected forward optical output. However, the power of the reflected forward optical output is more than three times that of the backward optical output, so the forward-originated optical output dominates. Conventional internal PD systems indirectly monitor the forward optical output using the backward optical output, requiring a constant ratio between the power of the backward optical output and the power of the forward optical output of the semiconductor laser chip. In contrast, the power of the forward optical output is directly monitored, making the forward-originated optical output dominant and less susceptible to the influence of the backward-originated optical output. This eliminates the need to ensure a constant ratio between the power of the backward optical output and the power of the forward optical output of the semiconductor laser chip. In addition, the power of the reflected forward optical output is determined by the reflectivity of the glass window, but the reflectivity of the glass window is more stable over the long term than the reflective film formed on the end facet of the semiconductor laser chip. Therefore, the above configuration improves the long-term stability of the output power compared to conventional internal PD systems.

[0018] In one embodiment, the power of the backward optical output incident on the light receiving element may be 1 / 10 or less of the power of the reflected forward optical output incident on the light receiving element, thereby further reducing the effect of long-term fluctuations in the ratio of the backward optical output power to the forward optical output power of the semiconductor laser chip, thereby improving long-term stability.

[0019] In one embodiment, the reflectance R of the glass window may be 2% or more. R≧2% This makes it easier to satisfy the design condition that the power of the backward optical output incident on the light receiving element is 1 / 10 or less of the power of the reflected forward optical output incident on the light receiving element for combinations of typically assumed package sizes and semiconductor laser chips with various specifications.

[0020] In one embodiment, a reflective coating may be formed on both sides of the glass window, on the inner surface of the glass window, or on the outer surface of the glass window.

[0021] In one embodiment, the reflectivity of the rearward reflection film formed on the rear end face of the semiconductor laser chip may be 99.5% or more, which makes it easier to satisfy the design condition that the power of the rearward optical output incident on the light receiving element is 1 / 10 or less of the power of the reflected forward optical output incident on the light receiving element for combinations of typically assumed package sizes and semiconductor laser chips with various specifications.

[0022] In one embodiment, the reflectivity of the rear reflection film formed on the rear facet of the semiconductor laser chip may be 99.9% or more, which makes it easier to satisfy the above design conditions.

[0023] In one embodiment, when the semiconductor laser device is viewed along the direction of emission of the forward optical output, the light receiving element may be offset from the semiconductor laser chip, thereby reducing the power of the backward optical output incident on the light receiving element.

[0024] In one embodiment, the semiconductor laser chip may include an absorption layer formed on a rear reflection film formed on the rear facet, which can reduce the power of the rear optical output incident on the photodetector.

[0025] In one embodiment, the absorbing layer may include a high refractive index layer or a metal film with a large extinction coefficient.

[0026] In one embodiment, the transmittance of the absorbent layer may be 1% or less.

[0027] (Embodiments) The present disclosure will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, parts, and processes shown in each drawing will be given the same reference numerals, and redundant explanations will be omitted as appropriate. Furthermore, the embodiments are examples rather than limitations on the disclosure, and all features and combinations thereof described in the embodiments are not necessarily essential to the disclosure.

[0028] The dimensions (thickness, length, width, etc.) of each member shown in the drawings may be enlarged or reduced as appropriate for ease of understanding. Furthermore, the dimensions of multiple members do not necessarily represent the relative size of each other. Even if a member A is depicted as being thicker than another member B in the drawings, member A may actually be thinner than member B.

[0029] 1 is a cross-sectional view of a semiconductor laser device 100A according to embodiment 1. The semiconductor laser device 100A is a CAN package laser, and includes an LD chip 110, a submount 120, a metal stem 130, a metal cap 140, a glass window 150, leads 160, and a light receiving element 170.

[0030] The LD chip 110 is an edge-emitting laser that emits a forward optical output from a front end face S1. The LD chip 110 is mounted on a submount 120, which is fixed to a metal stem 130. The oscillation wavelength of the LD chip 110 is not particularly limited, but the technology disclosed herein is useful in the oscillation wavelength band of GaN / InGaN-based lasers. A front reflective film 112 with a relatively low reflectivity is formed on the front end face S1 side of the LD chip 110, and a rear reflective film 114 with a relatively high reflectivity is formed on the rear end face S2 side.

[0031] The metal cap 140 covers the LD chip 110 from the side and supports and fixes the glass window 150. A reflective film 152 that reflects a portion of the forward light output L1 is formed on the glass window 150. The reflective film 152 may be formed on both sides of the glass window 150, or may be formed on only one side. The reflectance of the glass window 150 on which the reflective film 152 is formed is defined as Rwin. Here, the transmittance of the glass window 150 can be expressed as (1-Rwin).

[0032] A light-receiving element (internal PD) 170 is provided on the rear end face S2 side of the LD chip 110 so as to receive the reflected forward light output L1 reflected by the reflective film 152. The light-receiving element 170 is, for example, a Si photodiode. A typical metal stem 130 has an inclined surface (counterbore surface) 132 formed such that a light-receiving surface 172 of the light-receiving element 170 is non-parallel to the rear end face S2 of the LD chip 110. This is to prevent the backward light output emitted from the rear end face S2 of the LD chip 110 from being reflected by the light-receiving element 170 and returning to the LD chip 110.

[0033] The plurality of leads 160 penetrate the metal stem 130 while being insulated from the metal stem 130. The LD chip 110 and the light receiving element 170 are electrically connected to the corresponding leads 160. The plurality of leads 160 are electrically connected to a drive circuit (not shown).

[0034] The above is the configuration of the semiconductor laser device 100A. Next, the operation thereof will be described.

[0035] Fig. 2 is a diagram for explaining the operation of the semiconductor laser device 100A of Fig. 1. Light beams L1 to L6 are shown in Fig. 2. In Fig. 2, some members that are not related to the explanation of the light beams L1 to L6 are omitted.

[0036] A forward optical output L1 is emitted from the front end surface S1 of the LD chip 110. The power of the forward optical output L1 is denoted by Pf. Most of the forward optical output L1 passes through the glass window 150 and is extracted to the outside as output light L2 of the semiconductor laser device 100A. The power Po of the output light L2 is expressed by equation (2): Po=Pf×(1−Rwin) (2), where (1−Rwin) represents the transmittance of the glass window 150.

[0037] A portion of the forward light output L1 is reflected by the reflective film 152 of the glass window 150. The portion of the forward light output L1 reflected by the reflective film 152 is referred to as a reflected forward light output L3, and its power is represented by Pf'. Pf'=Pf×Rwin (3)

[0038] A portion L4 of the reflected forward light output L3 is incident on the light receiving surface 172 of the light receiving element 170. The power of the light L4 incident on the light receiving surface 172 is defined as Pf". In the following description, Pf" / Pf' is referred to as the light receiving rate ηf. The light receiving rate ηf is the rate of the light L4 incident on the light receiving surface 172 of the light receiving element 170 out of the reflected forward light output L3. Pf"=Pf'×ηf (4)

[0039] Furthermore, a portion L6 of the backward optical output L5 emitted from the rear end face S2 of the LD chip 110 is incident on the light receiving surface 172 of the light receiving element 170. When the power of the backward optical output L5 is Pr and the power of the incident light L6 is Pr', Pr' / Pr is defined as the light receiving rate ηr. Pr'=Pr×ηr (5)

[0040] An incident light L4 from the front and an incident light L6 from the rear are incident on the light receiving surface 172 of the light receiving element 170. The light receiving element 170 generates an electrical signal corresponding to the total power (referred to as detection power) Pdet of the two incident lights L4 and L6. Pdet=Pf″+Pr′ (6)

[0041] In a conventional semiconductor laser device with an internal PD, of the light incident on the light receiving element 170, the light L6 coming from behind is a signal component, and the rest are noise components. In contrast, in this embodiment, the light incident on the light receiving element 170 is dominated by the incident light L4 coming from the front, which becomes the main signal component. In other words, the semiconductor laser device 100A is configured to be able to monitor the output light L2 mainly based on the light L4 coming from the front.

[0042] The fact that the incident light L4 is dominant means that the power Pf" of the reflected forward light output L4 is three times or more the power Pr' of the rearward light output L6. In other words, the power Pr' of the rearward light output L6 incident on the light receiving element 170 is one-third or less of the power Pf" of the reflected forward light output L4 incident on the light receiving element 170.

[0043] The above is the operation of the semiconductor laser device 100A. Next, its advantages will be described.

[0044] In this semiconductor laser device 100A, a forward light output L1 is reflected by a reflective film 152 formed on a glass window 150, and incident light L4 based on the reflected forward light output L3 is detected by a light receiving element 170.

[0045] Substituting equations (1) to (5) into equation (6) yields equation (7). Pdet=Pf"+Pr'=Pf×Rwin×ηf+Pr×ηr (7)

[0046] Assuming that the power Pf″ of the reflected forward light output L4 is sufficiently larger than the power Pr′ of the backward light output L6 (Pf′ >> Pr′) on the light receiving surface 172 of the light receiving element 170, the following equation (8) is obtained: Pdet≈Pf×Rwin×ηf (8)

[0047] For example, it is preferable that the power Pr' of the incident light L6 incident on the light receiving surface 172 of the light receiving element 170 is 1 / 10 (i.e., 0.1) or less of the power Pf" of the reflected forward light output L4 incident on the light receiving surface 172. Pr' / Pf"≦0.1 ... (9) This reduces the dependency of the detection power Pdet on the light L6 coming from behind, making it less susceptible to fluctuations and variations in the reflectivity Rr of the rear end face S2 of the LD chip 110.

[0048] As can be seen from equation (8), the detected power Pdet is proportional to the reflectance Rwin and the light-receiving efficiency ηf, but the light-receiving efficiency ηf can be considered essentially a constant because it is determined geometrically. Furthermore, since the reflective film 152 is formed on the glass window 150, which has stable physical and optical properties, its optical properties are significantly more stable than those of the front reflective film 112 on the front end face S1 and the rear reflective film 114 on the rear end face S2, which are formed on semiconductors. Therefore, the reflectance Rwin is also more stable than the reflectances Rf and Rr of the LD chip 110. Therefore, in this embodiment, the power Pf of the forward light output L1 can be monitored with high accuracy by utilizing the reflected forward light output L3.

[0049] When feedback control is performed by an external driving circuit of the semiconductor laser device 100 so that the detected power Pdet is constant, the power Pf of the forward light output L1 is kept constant, and thus the power Po of the external light output L2 expressed by equation (2) is kept constant.

[0050] According to this embodiment, even if fluctuations in the LD characteristics (changes in Pr / Pf due to facet degradation), such as fluctuations in the facet reflectivity Rf or Rf, occur during long-term power-on operation, the ratio of the package's external optical output L2 to the internal PD monitor value is stably maintained, enabling accurate APC operation.

[0051] (Embodiment 2) Fig. 3 is a cross-sectional view of a semiconductor laser device 100B according to embodiment 2. Fig. 4 is a plan view of the semiconductor laser device 100B of Fig. 3. When the semiconductor laser device 100B is viewed from the emission direction of the forward optical output L1 of the LD chip 110, the light receiving element 170 is disposed offset in the Z direction with respect to the LD chip 110. In Fig. 4, a dashed line L4 indicates the range irradiated with the backward optical output L4.

[0052] According to the second embodiment, the light-receiving rate ηr for the backward light output L5 is lowered, so that the incident light L6 incident on the light-receiving surface 172 can be reduced. Conversely, the light blocked by the LD chip 110 for the reflected forward light output L3 is reduced, so that the light-receiving rate ηf is higher, so that the incident light L4 can be increased. This allows the ratio Pr' / Pf" to be reduced.

[0053] 5 is a cross-sectional view of a semiconductor laser device 100C according to a third embodiment. In the semiconductor laser device 100C, an absorption layer 116 is formed on a rear reflection film 114 at the rear end face S2 of the LD chip 110. The transmittance of the absorption layer 116 is set to 10% or less, preferably 5% or less, and more preferably 1% or less. The absorption layer 116 may be a high refractive index layer such as amorphous Si, or a metal film with a large extinction coefficient such as Au (gold).

[0054] By providing the absorption layer 116, the power of the backward optical output L6 incident on the light receiving surface 172 of the light receiving element 170 can be significantly reduced. This makes it possible to reduce the proportion of Pr' in the detected power Pdet and increase the proportion of Pf".

[0055] 6 is a cross-sectional view of a semiconductor laser device 100 simplified for design purposes. The dimensions are as follows: Lc: cavity length of the LD chip 110 Lf: distance from the front end face S1 of the LD chip 110 to the reflecting surface of the glass window 150 Lr: distance from the rear end face S2 of the LD chip 110 to the inclined face 132 within the plane 134 that is in contact with the bottom face of the submount 120 α: inclination angle of the inclined face 132 D: height to the waveguide (active layer) of the LD chip 110 based on the plane 134 that is in contact with the bottom face of the submount 120 d: height to the light-receiving surface 172 based on the inclined face 132 Ls: distance from the center of the light-receiving surface 172 of the light-receiving element 170 in the direction along the inclined face 132, and is the separation distance (shift amount) from the axis that passes through the center of the waveguide of the LD chip 110 along the waveguide of the LD chip 110 H: vertical width of the light-receiving surface 172 of the light-receiving element 170 Width of the light receiving surface 172 of the light receiving element 170

[0056] The following dimensions were used: D = 0.3 mm, d = 0.3 mm, α = 13°, Lf = 0.5 mm, Lr = 0.3 mm, 0.5 mm, 0.7 mm, Lc = 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, 1.5 mm, 2.0 mm, H = 0.5 mm, L = 0.5 mm

[0057] The forward light output L1 and the backward light output L5 emitted from the LD chip 110 are each assumed to have a Gaussian profile, with the beam's vertical (Z-direction) divergence angle θv being 22° and the beam's horizontal (Y-direction) divergence angle θh being 9°.

[0058] FIG. 7 is a diagram showing the powers Pf″ and Pr′ of light incident on the light receiving element 170 when Rwin=5%. The four graphs correspond to Rr=90%, 99%, 99.5%, and 99.7%. Rf=10% is common to all. The vertical axis shows the relative power normalized with the forward light output L1 set to 1 on a logarithmic axis. The horizontal axis shows the position of the light receiving element 170 (Ls in FIG. 6).

[0059] In each figure, Pf"×0.33 and Pf"×0.1 are also shown. When Rr=90%, the design condition Pr'<Pf"×0.1 is satisfied in the range of Ls≧0.34 mm, and Pr'<Pf"×0.33 is satisfied in the range of Ls≧0.32 mm. When Rr=99%, the design condition Pr'<Pf"×0.1 is satisfied in the range of Ls≧0.26 mm, and Pr'<Pf"×0.33 is satisfied in the range of Ls≧0 mm. In other words, when the design condition Pr'<Pf"×0.33 is met, shifting of the light receiving element 170 is unnecessary. When Rr=99.5%, the design condition Pr'<Pf"×0.1 is satisfied in the range of Ls≧0.20 mm. When Rr=99.7%, the design condition Pr′<Pf″×0.1 is satisfied in the range of Ls≧0 mm. In other words, there is no need to shift the light receiving element 170.

[0060] Assume that the sensitivity of a PD typically used as the light receiving element 170 is 0.16 A / W. In order to ensure an output current of 50 μA from the light receiving element 170 when the external optical output L2 is operating at a power Po=60 mW, Pf″>0.005 is required. Therefore, under this constraint, the upper limit of the shift amount Ls of the light receiving element 170 is 0.68 mm.

[0061] 8 is a diagram showing the powers Pf" and Pr' of light incident on the light receiving element 170 when Rwin = 3%. The four graphs correspond to Rr = 90%, 99%, 99.5%, and 99.9%. Rf = 10% is common to all. When Rr = 90%, the design condition Pr' < Pf" x 0.1 is met in the range of Ls > 0.36 mm. When Rr = 99%, the design condition Pr' < Pf" x 0.1 is met in the range of Ls ≥ 0.28 mm. When Rr = 99.5%, the design condition Pr' < Pf" x 0.1 is met in the range of Ls ≥ 0.26 mm. When Rr = 99.9%, the design condition Pr' < Pf" x 0.1 is met in the range of Ls ≥ 0 mm. In other words, there is no need to shift the light receiving element 170.

[0062] The upper limit of the shift amount Ls of the light receiving element 170 that satisfies the condition Pf">0.005 for ensuring the output current of the PD is 0.54 mm.

[0063] Furthermore, when the reflectance Rwin of the glass window 150 is reduced, the reflectance Rr of the rear end surface S2, which can make the shift amount Ls of the light receiving element 170 zero, increases.

[0064] FIG. 9 is a diagram showing the powers Pf″ and Pr′ of light incident on the light receiving element 170 when Rwin=2%. The four graphs correspond to Rr=90%, 99%, 99.5%, and 99.9%. Rf=10% is common to all graphs.

[0065] When Rr=90%, the design condition Pr'<Pf"×0.1 is satisfied in the range of Ls>0.38 mm. When Rr=99%, the design condition Pr'<Pf"×0.1 is satisfied in the range of Ls≧0.30 mm. When Rr=99.5%, the design condition Pr'<Pf"×0.1 is satisfied in the range of Ls≧0.28 mm. When Rr=99.9%, the design condition Pr'<Pf"×0.1 is satisfied in the range of Ls≧0 mm. In other words, there is no need to shift the light receiving element 170.

[0066] Furthermore, the upper limit of the shift amount Ls of the light receiving element 170 that satisfies the condition Pf″>0.005 for ensuring the output current of the PD is 0.40 mm.

[0067] FIG. 10 shows the powers Pf″ and Pr′ of light incident on the light receiving element 170 when Rwin=1%. The four graphs correspond to Rr=90%, 99%, 99.5%, and 99.9%. Rf=10% is common to all. If Rwin is reduced to 1%, the condition Pf″>0.005 for ensuring the output current of the PD cannot be met.

[0068] The following findings can be obtained from a comparison of Figures 7 to 10. - When the reflectance Rwin of the glass window 150 is lowered, the shift amount Ls of the light receiving element 170 required to satisfy the design conditions increases. - When the reflectance Rwin of the glass window 150 is lowered, the upper limit of the shift amount of the light receiving element 170 also decreases. The lower the reflectance Rwin, the narrower the range in which the light receiving element 170 can be arranged. In other words, the higher the reflectance Rwin, the wider the range in which the light receiving element 170 can be arranged.

[0069] Considering the normally possible facet reflectivities Rf and Rr of the LD chip 110 and the shift amount Ls of the light receiving element 170, it is preferable to set Rwin ≥ 2%, and setting Rwin ≥ 3% increases the degree of freedom in designing other parameters. Furthermore, if Rwin is too high, the forward output will be low, so Rwin ≤ 30% is desirable.

[0070] Furthermore, the realistic distance Lr between the LD chip 110 and the inclined surface 132 is about 0.5 mm. Assuming that Lr≈0.5 mm, if the design satisfies Rwin≧3% and Rr≧99.93%, the above-mentioned design conditions can be generally met for various specifications of the LD chip 110.

[0071] (Design of Glass Window 150) Next, the reflectance Rwin of the glass window 150 will be described.

[0072] 11 is a cross-sectional view of the glass window 150. The reflectance of the inner surface of the glass window 150 is Ri, and the reflectance of the outer surface is Ro. The effective reflectance Rwin of the glass window 150 is expressed by equation (10): Rwin=(Ri+Ro-2Ro.Ri) / (1-Ri.Ro) (10)

[0073] For example, when Ro<<Ri, Equation (10) can be approximated by Equation (11): Rwin≈Ri+Ro(1−Ri) ２ ...(11) Furthermore, when Ro is small (for example, Ro<0.3%), it can be approximated by equation (12). Rwin ≈ Ri ...(12)

[0074] Therefore, it may be designed so that Ri≧3% and Ro<0.3%. In this case, a reflective film 152 may be formed on the inner surface of the glass window 150, and an anti-reflective film (AR coating) 154 may be formed on the outer surface of the glass window 150.

[0075] The material of the glass window 150 is SiO ２ In this case, the anti-reflection film 154 may be designed to have a four-layer structure as shown below: ２ 42nm 3rd layer TiO ２ 67nm 2nd layer SiO ２ 54nm 1st layer TiO ２ 74nm glass window SiO ２

[0076] This anti-reflection film 154 can achieve a reflectance of 0.1% or less in the practical wavelength band of 395 to 415 nm. Therefore, the reflectance Rwin can be designed by taking into consideration only the inner reflective film 152.

[0077] The reflective film 152 and the anti-reflective film 154 may be switched from the inside to the outside to satisfy the conditions Ri<<Ro, Ro≧3%, and Ri<0.3%.

[0078] When Ri = Ro = R, the reflectance Rwin is expressed by equation (13): Rwin = 2R / (1 + R) (13) Calculating equation (13) backwards for R yields equation (14): R = Rwin / (2 - Rwin) (14) Therefore, when the design value of Rwin is Rd, the reflectance 152 may be designed to satisfy relational equation (15): Ro = Ri = Rd / (2 - Rd) (15) Note that in equation (14), if Rwin is 10% or less, it can be assumed that (2 - Rwin) ≈ 2 within an accuracy of 5%, and equation (13) can be approximated by the following equation (13'): Rwin ≈ 2R (13') Therefore, when the design value of Rwin is Rd, the reflective film 152 may be designed to satisfy Ro = Ri = Rd / 2.

[0079] Furthermore, when Ro≠Ri and Ro is set to a fixed value in advance, reverse calculation of formula (10) with respect to Ri gives formula (16): Ri=(Rwin-Ro) / (1-2 Ro+Rwin Ro) (16) Therefore, when Ro is set to an arbitrary fixed value and the design value of Rwin is set to Rd, reflectance film 152 may be designed to satisfy relational formula (17): Ri=(Rd-Ro) / (1-2 Ro+Rd Ro) (17) Note that when Ri is set to a fixed value in advance instead of Ro, reverse calculation of formula (10) with respect to Ro gives formula (18). Ro=(Rwin-Ri) / (1-2*Ri+Rwin*Ri) (18) Therefore, when Ri is an arbitrary fixed value and the design value of Rwin is Rd, the reflective film 154 may be designed to satisfy the relational expression (19). Ro=(Rd-Ri) / (1-2*Ri+Rd*Ri) (19) Furthermore, when Ro or Ri is fixed in advance, for example, when Ro is fixed and Ro≦5% and Rwin<2Ro / (1+Ro)≦9.5%, Ri<Ro≦5% is satisfied, and therefore, expression (10) can be approximated by expression (10) within an accuracy of 5%. Rwin≒Ri+Ro ... (10') Therefore, when the design value of Rwin is set to Rd smaller than 9.5%, the reflective film 152 and the reflective film 154 may be designed to satisfy Ri≦5%, Ro≦5%, and Rd=Ri+Ro.

[0080] 12 is a cross-sectional view of an LD chip 110 according to one embodiment. As described above, a high reflectivity Rr is required for the rear reflective film 114 of the LD chip 110. The rear reflective film 114 can be formed of a dielectric multilayer film.

[0081] The rear reflection film 114 may include an underlayer 114a made of AlN and a dielectric multilayer film 114b. Assuming that the oscillation wavelength band of the LD chip 110 is 395 to 415 nm, the dielectric multilayer film 114b may be made of SiO ２ layer and TiO ２ It can be a multilayer structure of SiO ２ When the refractive index n is 1.49, SiO ２The layer thickness is 67 nm. ２ When the refractive index n is 2.82, TiO ２ The dielectric multilayer film 114b is a SiO 2 film having a thickness of λ / 2n (=134 nm). ２ The layer is terminated.

[0082] 13 is a diagram showing the reflectance Rr and transmittance of the rear end face of the LD chip 110 in FIG. ２ layer and TiO ２ The thickness of the underlying AlN layer is 20 nm.

[0083] 14 is a cross-sectional view of an LD chip 110 according to one embodiment. In this LD chip 110, an absorption layer 116 is formed on a dielectric multilayer film 114b made of N layers. The absorption layer 116 is a SiO ２ The absorbing layer 116 includes a layer 116a, a high-refractive-index layer 116b, and a termination layer 116c. The high-refractive-index layer 116b has a thickness of 3λ / 4. A material with a refractive index of 4 or more can be selected for the high-refractive-index layer 116b, and a-Si (amorphous silicon) with a refractive index of 5.42 is suitable, for example. The transmittance of the absorbing layer 116 should be 1% or less, which allows the reflectance of the dielectric multilayer film 114b to be low, or in other words, allows the number of pairs N to be designed to be small.

[0084] Fig. 15 is a diagram showing the transmittance of the rear end facet of the LD chip 110 of Fig. 14. For comparison, the left side of Fig. 15 shows the transmittance when the high refractive index layer 116b is not included. Adding the high refractive index layer 116b has the same effect as adding two pairs of dielectric multilayer films.

[0085] 16 is a cross-sectional view of an LD chip 110 according to one embodiment. The absorption layer 116 of this LD chip 110 includes a metal layer 116d. For the metal layer 116d, a material with a large extinction coefficient k is preferably selected, such as Au with k=1.96.

[0086] Fig. 17 is a diagram showing the transmittance of the rear facet of the LD chip 110 of Fig. 16. For comparison, the left side of Fig. 17 shows the transmittance when the metal layer 116d is not included. Adding the metal layer 116d has the same effect as adding three pairs of dielectric multilayer films.

[0087] The embodiments merely illustrate the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments as long as they do not deviate from the spirit of the present invention as defined in the claims.

[0088] 100 Semiconductor laser device 110 LD chip 112 Front reflection film 114 Rear reflection film 114a Underlayer 114b Dielectric multilayer film 116 Absorption layer 116a SiO2 layer 116b High refractive index layer 116c Termination layer 116d Metal layer 120 Submount 130 Metal stem 132 Inclined surface 134 Reference plane in contact with the bottom surface of the submount 140 Metal cap 150 Glass window 152 Reflection film 160 Lead 170 Light receiving element 172 Light receiving surface S1 Front end face S2 Rear end face L1 Forward optical output L2 External optical output L3 Reflected forward optical output L4 Incident light L5 Rear optical output L6 Incident light

Claims

1. A semiconductor laser device comprising: an edge-emitting semiconductor laser chip that radiates a forward optical output from its front end face; a glass window provided on the front end face side of the semiconductor laser chip and that reflects a portion of the forward optical output; and a light-receiving element provided on the rear end face side of the semiconductor laser chip so as to receive a reflected forward optical output, which is the forward optical output reflected by the glass window; wherein the power of the rear optical output radiated from the rear end face of the semiconductor laser chip and incident on the light-receiving element is 1 / 3 or less of the power of the reflected forward optical output incident on the light-receiving element.

2. A semiconductor laser device according to claim 1, wherein the power of said backward optical output incident on said light receiving element is 1 / 10 or less of the power of said reflected forward optical output incident on said light receiving element.

3. A semiconductor laser device as claimed in claim 1, characterized in that the reflectance Rwin of said glass window is 2% or more.

4. A semiconductor laser device according to any one of claims 1 to 3, characterized in that the reflectance of the rear reflection film formed on the rear end face of the semiconductor laser chip is 99.5% or more.

5. A semiconductor laser device according to claim 4, wherein the reflectance of the rear reflection film formed on said rear end face of said semiconductor laser chip is 99.9% or more.

6. A semiconductor laser device as described in any one of claims 1 to 3, characterized in that, when the semiconductor laser device is viewed along the emission direction of the forward optical output, the photodetector is arranged offset with respect to the semiconductor laser chip.

7. A semiconductor laser device according to any one of claims 1 to 3, characterized in that said semiconductor laser chip comprises an absorption layer formed on a rear reflection film formed on said rear end face.

8. A semiconductor laser device according to claim 7, wherein the transmittance of said absorption layer is 1% or less.

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