JPH05259538A - Optical waveguide ring laser - Google Patents

Abstract

PURPOSE:To obtain an optical waveguide ring laser capable of controlling arbitrarily an oscillation wavelength by a method wherein a wavelength selecting element, which transmits light of a specified wavelength, is provided in a ring laser resonator. CONSTITUTION:The so-called narrow-band transmitting filter, which transmits light of a specified wavelength and stops light other than the light of a specified wavelength, is added in a ring laser resonator 14 constituted of an Er-doped optical waveguide as a waveguide selecting element 15. Thereby, a resonator circuital loss in a wavelength other than that in the transmitting band of the light is increased and it becomes possible to make a laser oscillation take place in an arbitrary wavelength to coincide with the transmitting wavelength of the element 15. As the narrow-band transmitting filter, a dielectric multilayer film filter, which is superior in the controllability of a transmitting wavelength and a transmitting band, is practical. It is desirable that the element 15 is arranged in a position immediate before an excitation light circuits in the resonator 14 and is emitted through an excitation light incident port or is arranged in the optically rear position of the Er-doped optical waveguide part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to optical communication, optical information processing,
The present invention relates to a structure of an optical waveguide type rare earth ion-doped glass ring laser which is highly useful as a light source in fields such as optical measurement.

[0002]

2. Description of the Related Art A silica-based optical waveguide ring laser in which a rare earth ion such as Er, Nd or Pr is added to a core can obtain an oscillation threshold of a small excitation light intensity and a high conversion efficiency, and an oscillation line width. Is narrow and the vertical mode interval can be precisely controlled. Therefore, various applications are expected in a wide range of fields such as optical communication, optical information processing, and optical measurement. In particular, an optical waveguide laser with Er ions added as active ions is important for optical communication.
Since it has a light emitting band in the m band, it is extremely useful. Er
Ions are in the 0.82 μm band, 0.98 μm band and 1.4
It has an absorption band in the near-infrared region such as 8 μm band, is excited by light having a wavelength suitable for these, and stimulates and emits light in the 1.5 μm band.

Conventional optical waveguide type ring lasers are roughly classified into two types, a pumping light through type and a pumping light cross type, depending on the wavelength characteristics of the directional coupler for introducing pumping light and oscillating light. The pumping light through type directional coupler is suitable for a manufacturing process of a silica-based optical waveguide circuit, and can achieve a required coupling rate characteristic with a high yield. When this is used as an input / output port of a ring laser, it is necessary to incorporate a cross waveguide in the ring resonator. FIG. 4 shows the structure of an optical waveguide type ring laser having a pumping light through type directional coupler invented by the present applicants (Japanese Patent Application No. 3-281579). This ring laser has a ring laser resonator 45 formed in a flat substrate, a directional coupler 43 used for introducing pumping light into the resonator 45 and deriving oscillated light from the resonator, and an intersecting portion 44. .. Directional coupler 43
Has a coupling wavelength characteristic that the coupling rate is small at the excitation light wavelength and the coupling rate is large at the oscillation wavelength in the 1.5 μm band.

When pumping light enters from one waveguide end face 41 of this optical waveguide type laser, the pumping light is directional coupler 4
3 is introduced into the ring laser resonator 45 to excite Er ions in the resonator 45. When the amplification degree based on the stimulated emission of excited Er ions becomes larger than the circulating light loss of the ring laser resonator 45, the ring laser resonator 45 starts oscillating, and 1.5 μ derived through the directional coupler 43.
Laser oscillation light in the m band is output from the other oscillation light output port 47.

[0005]

The Er ion, which operates as a three-level system, has a broad absorption band and an emission band in the 1.5 μm band, each of which is a fine particle corresponding to Stark splitting of the energy level. It has a structure. Further, there is a feature that the wavelength characteristic of the amplification degree changes depending on the degree of population inversion. When the excitation light intensity is low, the amplification degree is large at 1.60 μm, which corresponds to the fine structure on the long wavelength side of the 1.5 μm band, and when the excitation light intensity is high, it is the emission peak wavelength of the entire 1.5 μm band. The amplification degree becomes maximum at 1.535 μm. In the Er ion-doped optical waveguide type ring laser, laser oscillation is started at a wavelength and pumping light intensity at which the circulating light loss of the ring laser resonator and the amplification degree match, so that Er
The oscillating wavelength is 1.53 due to the added concentration and the loss of light from the resonator
It has been difficult to oscillate at a local emission peak wavelength of Er ions such as 5 μm or 1.60 μm and arbitrarily control the oscillation wavelength. Therefore, development of an optical waveguide type ring laser that oscillates at an arbitrary wavelength in the 1.5 μm band has been awaited. Therefore, an object of the present invention is to provide a structure of an optical waveguide type ring laser capable of arbitrarily controlling the oscillation wavelength.

[0006]

In order to solve the above problems, the present invention provides an optical waveguide ring laser in which a ring laser resonator is formed on a flat substrate by an optical waveguide in which at least a part of rare earth ions is added. And transmitting at least one light of a specific wavelength into the ring laser resonator.
One wavelength selecting element is provided.

[0007]

A so-called narrow band transmission filter, which transmits light of a specific wavelength and blocks light of other wavelengths, is added as a wavelength selection element in a ring laser resonator constituted by an Er-doped optical waveguide to obtain a transmission region. It becomes possible to increase the round-trip loss of the resonator at wavelengths other than, and to oscillate at an arbitrary wavelength that matches the transmission wavelength of the wavelength selection element.

[0008]

EXAMPLES The present invention will be described in detail below with reference to examples. FIG. 1 shows the structure of an optical waveguide type ring laser according to the present invention. Reference numeral 12 is a waveguide for introducing pumping light, 13 is a directional coupler for inputting pumping light and oscillating light, 14 is a ring laser resonator composed of an Er-doped optical waveguide, and 15 is a narrow band transmission filter used as a wavelength selection element. , 16 are waveguides for deriving oscillated light. In FIG. 1, the ring laser resonator is shown focusing on the optical path of the oscillated light, so that the crossing portion of the conventional ring laser illustrated above is not shown.

As the narrow band transmission filter, a dielectric multilayer filter having excellent controllability of transmission wavelength and transmission band is practical. The transmission wavelength of the multilayer filter can be precisely controlled by the refractive index and the film thickness of the multilayer material, and the transmission wavelength band can be widely changed depending on the number of layers of the multilayer filter. It is the oscillation wavelength of the Er-doped optical waveguide laser.
In the 5 μm band, a bandpass filter having a transmission wavelength range of 0.1 nm and an extremely narrow band is put into practical use.

The wavelength selection element 15 is connected to the ring laser resonator 1
4, the groove 23 is formed by etching or machining so as to traverse the optical waveguide 21 forming the ring laser resonator, and the groove 23 is formed on the filter substrate 25 in the groove 23 as shown in FIG. The formed narrow band transmission dielectric multilayer filter 24 is provided. By changing the transmission wavelength of the dielectric multilayer film bandpass filter 24, the wavelength of 1.5
Laser oscillation can be realized with good controllability in a wide wavelength range from 3 μm to 1.60 μm. When the reflected light from the surface of the dielectric multilayer filter 24 is recombined with the waveguide, a Fabry-Perot resonator in which one multilayer filter is shared as both end face mirrors is configured, and a blocking wavelength having a high reflectance is used instead of a transmission wavelength. Laser oscillation. In order to prevent this, it is necessary to incline the filter surface with respect to the waveguide so that the reflected light does not return to the optical waveguide. By setting the angle between the tangent line of the waveguide and the normal line of the filter surface to be 1 ° or more, the reflection from the filter surface can be sufficiently suppressed.

The loss of the oscillated light in the wavelength selection element 15 is the loss in the transmission wavelength of the narrow band transmission filter, and the groove 2
It is given by the sum of the diffraction loss and the scattering loss at the end face of the waveguide when spatially propagating in 3. In order to construct the wavelength selection element 15 with a small insertion loss, it is important to use a high quality multilayer film filter with a small loss at the transmission wavelength. Moreover, as a result of the beam diameter expanding when the beam emitted from the end face of the waveguide propagates through the groove 23 in space, only a part of the light is recombined with the waveguide after passing through the multilayer filter, resulting in an optical loss. Become. In order to reduce such diffraction loss, the width of the groove 23 corresponding to the distance of propagation in free space is 0.1 mm.
The following is desirable, and it is necessary that the inserted multilayer filter is thin. As a thin multilayer filter, for example, a thickness of 3 formed on a polyimide film or a quartz thin plate.
A multilayer filter of about 0 μm has been developed and can be used. By embedding and fixing the groove 23 and the inserted filter with the optical adhesive 26 having a refractive index matching with that of the optical waveguide, it is possible to reduce connection loss due to scattering and reflection of the end face and to secure long-term reliability.

When the divergence angle of the beam emitted from the optical waveguide is large, the insertion loss due to optical diffraction increases,
In addition, the transmission band of the narrow band filter is widened. In order to improve the efficiency of the laser by the high NA waveguide structure and at the same time reduce the filter insertion loss and prevent the expansion of the transmission band, for example, at the end portion of the optical waveguide 21 that allows light to enter and exit the wavelength selection element 15, It is effective to increase the mode field diameter by providing an up taper 22, a down taper, or a thermal diffusion portion of a refractive index control additive.

It is desirable that the wavelength selection element 15 is arranged at a position where the excitation light intensity is as small as possible. Since a practical multimode oscillation semiconductor laser is used as the excitation light source, it is practically impossible to realize a narrow band dielectric multilayer filter having a high excitation light transmittance. Therefore, as shown in FIG. 1, the excitation light circulates in the resonator 14 and is arranged at a position immediately before being emitted from the excitation light incident port, or a part of the ring resonator 14 is formed by an Er-doped waveguide. It is preferable that the constructed wavelength selecting element 15 is arranged at a position optically rearward of the Er-doped waveguide portion. Further, the same effect can be obtained even if there are two or more wavelength selection elements.

Example 1 An oscillation wavelength control experiment was carried out by incorporating a dielectric multilayer film filter in a ring resonator composed of an Er-doped silica optical waveguide.

FIG. 3 shows the structure of the ring laser used. The waveguide is a buried waveguide formed on a silicon substrate by the flame deposition immersion method and the reactive ion etching method. The concentration of Er ions added to the core is 1% by weight, the cross section of the core has a height of 5 μm width, a width of 5 μm, and the refractive index difference between the core and the cladding is 0.75%. The mode field diameter of the waveguide is 6 μm. The ring laser is a ring laser resonator 35 incorporating a wavelength selection element and having a circumference of 10 cm.
And a cross section 34, and a directional coupler 33 used for introducing pumping light into the resonator 35 and deriving oscillating light from the resonator 35. The directional coupler 33 has a small coupling rate of 20% at an excitation light wavelength of 0.98 μm and an oscillation wavelength of 1.
It has a large coupling wavelength characteristic with a coupling rate of 90% at 55 μm. As the wavelength selection element 39, a SiO ₂ —TiO ₂ system multilayer film filter (thickness 30 μm) deposited on a polyimide film was used.
m) was used. The transmission wavelength is 1.550 μm, the transmission band is 0.1 nm, and the loss at the transmission wavelength is 1 dB. The transmittance of the strained superlattice semiconductor laser light having a wavelength of 0.98 μm used as the excitation light is 3%. The narrow band transmission multilayer filter was arranged in the filter insertion groove 38 formed 2 cm before the directional coupler 33 in which the excitation light circulates in the resonator and is emitted from the excitation light incident port. The waveguides on both sides of the insertion groove 38 are uptapered to increase the mode field diameter at the end surface of the insertion groove to 10 μm. The filter insertion groove 38 was formed by cutting with a dicing saw. The groove 38 has a width of 40 μm and a depth of 0.2 mm. The angle between the tangent to the waveguide and the normal to the filter surface should be 5 °,
The angle between the waveguide tangent and the groove was set to 85 °. A narrow band transmission multilayer filter 39 was inserted into the groove 38 and fixed and held by an ultraviolet curable optical adhesive.

A laser oscillation experiment was conducted by injecting pumping light from the pumping light entrance port of the ring laser. As a result, a wavelength of 1.550μ that matches the transmission wavelength of the narrow band transmission filter.
It became clear that laser oscillation occurs at m. The characteristics of oscillation threshold of excitation light intensity of 30 mW and differential efficiency of 5% were obtained.

(Embodiment 2) Half the circumference of the ring resonator portion is constituted by an Er-doped optical waveguide and the rest is constituted by an Er-free optical waveguide, and the wavelength selection element is optically rearward of the Er-doped waveguide portion. The same experiment as in Example 1 was performed by arranging at the position of. As a result, it was revealed that laser oscillation occurred at a wavelength of 1.550 μm, which coincided with the transmission wavelength of the narrow band transmission filter, as in Example 1.

(Comparative Example) The same laser oscillation experiment as in Example 1 was carried out by removing the narrow band transmission multilayer filter from the ring laser used in Example 1 and completely filling the filter insertion groove 38 with an optical adhesive. I went. As a result, it was revealed that the laser oscillation wavelength was 1.60 μm, and that the laser oscillated at a wavelength different from the transmission wavelength of the narrow band transmission multilayer filter.

[0019]

As described above, the optical waveguide type ring laser of the present invention is an optical waveguide type ring laser in which a ring laser resonator is formed on a flat substrate by an optical waveguide in which at least a part of rare earth ions is added. Since the ring laser resonator is provided with at least one wavelength selection element that transmits light of a specific wavelength, the present ring laser allows the laser oscillation wavelength to be any wavelength in the 1.5 μm band. That is, it becomes possible to control with high accuracy to a wavelength equal to the transmission wavelength of the wavelength selection element.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an example of an optical waveguide ring laser according to the present invention.

FIG. 2 is a configuration diagram showing a method of installing a narrow band transmission dielectric multilayer filter as a wavelength selection element.

FIG. 3 is a schematic configuration diagram showing an optical waveguide type ring laser according to a first embodiment of the present invention.

FIG. 4 is a schematic configuration diagram showing a conventional optical waveguide ring laser.

[Explanation of symbols]

 14 Er-doped optical waveguide ring laser resonator 15 Wavelength selection element

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Keizo Suto, 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. An optical waveguide type ring laser in which a ring laser resonator is formed on a flat substrate by an optical waveguide to which rare earth ions are added at least in a part thereof, and light of a specific wavelength is transmitted through the ring laser resonator. An optical waveguide type ring laser provided with at least one wavelength selecting element for 2. The optical waveguide ring laser according to claim 1, wherein the wavelength selection element is arranged at a position optically rearward of the rare earth ion-doped optical waveguide portion in the ring laser resonator.