Technical Information on September 30th, 2023
Features of spatially coupled optical fiber compact functional devices
Shozo Tanaka
OPTOQUEST CO.,LTD
1. Introduction
In basic research on optics, especially using lasers as light sources, it has been a long timesince experimental systems using optical fibers have been adopted in many cases. Not only
the research subjects for optical communication fields, but also for sensing, quantum
computing, spectroscopy, measurement, and even high-end processing using lasers, it is very
convenient to use optical fibers to handle them, and the transverse mode is also stable,
therefor, good characteristics can be obtained.
Optoquest manufactures high end optical fiber compact functional devices as research andexperimental tools not only in the telecom field but also in various optical fields. Some of ourrecommended products are introduced in this paper.
2. Key technology is low-loss spatially coupled fiber optic in-out
It is not easy to couple the signal light with highly efficiency to the core of a generally used
quartz glass single-mode optical fiber in devices that perform some function in spatial optics.
By using aspherical lenses that are well designed and selected considering aberrations(Fig. 1), the fiber collimator pair (Fig. 2) can be fabricated with the guaranteed opticalcoupling loss value ≤ 0.5 dB and the typical loss value ≤ 0.25 dB by using precise optical axis
alignment equipment. In addition, our original YAG welded fixing technology (Fig. 3)
provides the same high reliability as passive optical function modules that have passed theTelcordia GR-1221 test.

Fig. 1: Characteristics of defocus and beam waist radius-1/12-Fig. 2: Schematic diagram of Fiber collimator pairFig. 3: Photograph of YAG laser welded fixing
3. Polarization Controller
Commercially available optical fiber polarization controllers can be classified into thefollowing three main types. (1) paddling type, (2) in-line stressing type, and (3) bulk type. In
general, the order of cheapness is (1), (2), and (3), and the order of performance is (3), (2), and(1). Here we introduce the bulk type polarization controller in (3), which is fabricated byOptoquest.
3.-1) Abstract
By rotating some bulk polarizing elements in the spatial beam of a fixed fiber collimatorpair (Fig. 4), any polarization states can be obtained according to the configuration of theelements.

Fig. 4: Schematic diagram of bulk type Polarization Controller
The bulk type has more repeatability of the polarization state than the paddling type andthe stressing type. Moreover, since this is a cartridge type, various experiments andinspections of the optical fiber system can be conveniently performed by simply inserting andremoving the respective cassettes of the polarizing elements and the optical functionalelements (Fig. 5).

-2/12-(a) Example: Polarization Controller(b) fixed fiber collimator pair(c) Polarizer Cassette, (d) Quarter Wave Plate Cassette, (e) Half Wave Plate CassetteFig. 5: (a)～(e): Photograph of cartridge type polarization controller and its component
cassettes
3.-2) Overview of Polarization Control Principles
The principle is easy to explain since it is a bulk type. The polarizer is made of wire-gridtype glass, and the QWP and HWP wave plates are made of quartz crystal. A polarizer
transmits only linear polarization along its axis. By rotating QWP, the polarized ellipticityof the transmitted polarization can be changed. By rotating HWP, the polarized axis
direction of the transmitted polarization can be changed.
・Role of the wave plates
A wave equation ‘S’ for linearly polarized light with amplitude ‘a’ (Fig. 6) is expressed asfollows.
S = a sin (ω・t)【’ω’ is angular velocity and ‘t’ is time】

Before the light ‘S’ enters the uniaxial crystal in the coordinate system of Fig. 6, it can beexpressed as follows.
X = a cosθ・sin (ω・t),
Y = a sinθ・sin (ω・t)-3/12-

Now, let us consider the case in which a light with amplitude ‘a’ is transmitted through an
optical crystal plate (for example, zero order quartz crystal waveplate) in Fig. 7
perpendicularly to the paper.
The linear polarization along the crystal axis is ‘extraordinary light’, and the refractive
index is ne. The linear polarization perpendicular to the crystal axis is ‘ordinary light’, andthe refractive index is no. In the case of quartz crystal, ne＞no.

The equation for a wave transmitted through a uniaxial crystal can be expressed as follows.
X = a cosθ・sin (ω・t + δo), Y = a sinθ・sin (ω・t + δe)
Since ne＞no for the quartz crystal, the phase difference is δo＞δe.
The phase difference δ can be expressed as δo −δe =δ.
Since δe = δo –δ, Y = a sinθ・sin (ω・t + δo − δ), so the phase of extraordinary lightis delayed by ‘δ’ compared to ordinary light.
Incidentally, the phase difference between ordinal light and extraordinary light can beexpressed as δ=2𝜋
𝜆・d・(ne − no). ‘d’ is the thickness of the quartz crystal plate.

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Some examples of polarization state that linear polarization can obtain before and after
transmission through the quartz crystal plate are shown below.
Example (1): θ = 45° and δ = 0
X=Y=𝑎・sin (ω・t + δo)

√2𝑎𝑎

√2

√2・sin (ω・t + δo −δ) =・sin (ω・t + δo)∴ Y=X→ Represents linear polarization before transmission
Example (2): θ = 45° and δ = π
X=
Y=𝑎・sin (ω・t + δo)

√2𝑎𝑎

√2

√2・sin (ω・t + δo −δ) =𝑎= −・sin (ω・t + δo − π)・sin (ω・t + δo)√2∴ Y= − X→ Represents linear polarization rotated 90°to the left after transmission
as shown Fig, 8𝜋Example (3): θ = 45° and δ = 2
X=Y==𝑎・sin (ω・t + δo)

√2
𝑎𝑎

√2𝑎

√2・sin (ω・t + δo −δ) =・sin (ω・t + δo – π / 2)・cos (ω・t + δo)√2∴ X2 + Y2 = a 2 / 2→ Represents clockwise circular polarization after transmission as shown Fig.
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Example (4): θ = 45° and δ = 3 2X=Y=𝑎・sin (ω・t + δo)

√2𝑎𝑎

√2

√2= −・sin (ω・t + δo −δ) =𝑎・sin (ω・t + δo – 3π / 2)・cos (ω・t + δo)√2∴ X2 + Y 2 = a 2 / 2→ Represents counterclockwise circular polarization after transmission
・Conclusion of the bulk type polarization controller
A polarizer transmits only linear polarization along its axis, so high extinction ratio canbe obtained.
By changing the angleθ of the polarized light axis incident on the HWP (i.e., δ=π), anarbitrary polarization axis direction of – 2θ can be obtained with a constant ellipticity as
shown in Fig. 10.
By changing the angleθ of the polarized light axis incident on the QWP (i.e., δ=π/2),
an arbitrary polarization ellipticity can be obtained with a constant polarization axis
direction as shown in Fig. 11.

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4. Variable Attenuator ModuleThis device is also a bulk type manually variable attenuator (Fig. 12 and Fig. 13) like thepolarization controller.
There are three features.
(1) Can withstand 2W optical power input because it does not use light-absorbing material
such as a knife edge in the optical path. It uses a prism.
(2) Equipped with an On/Off shutter. This also uses a prism.
(3) Variable attenuation uses a micrometer, so repeatability is very good.
Commercially available manually variable optical attenuators often use "screws" forattenuation. They require very careful fingering during frequent variable attenuation.
Since this device uses a micrometer for variable attenuation adjustment, you are free fromthe stress of fingerings. Of course, backlash is extremely low compared to the screw type.
Furthermore, PMF is in high demand for input and output fibers of this device.

Fig. 12: Schematic diagram of bulk type Variable Attenuator Module

Fig. 13: Photograph of Variable Attenuator Module-7/12-

5. Polarization Composition / Separation Coupler
This device is a 3-port module that can combine and separate polarizations (Fig. 14 andFig. 15) with high extinction ratio. Also called PBS module, it is a bulk type device using opticalcrystal. It is not a polarization splitting or branching coupler.
Some commercially available polarization couplers are (1) PM fiber fusion type and (2)PBS module type applying Brewster's law using multilayer interference film. As is well known,
the polarization separation ability of types (1) and (2) is limited to about 30 dB.
This device uses the uniaxial optical crystal which provides high polarization separation
extinction ratio of ＞45dB. Ideal for high performance polarization diversity applications. It isoften used as a polarization combination device for the purpose of TE and TM input to thepolarization-dependent devices.

Fig. 14: Photograph of Polarization Composition / Separation Coupler
The mechanism of this device is briefly shown below (Fig. 15 and Fig. 16). The uniaxial
optical crystal is used as a birefringent plate. Since a birefringent plate and a folding prism
are used, it’s a 3-port device with only one side.

Fig. 15: Schematic diagram of bulk type Polarization Composition / Separation Coupler-8/12-Fig. 16; Functional diagram of Birefringent plate
It is known that the separation width ‘d’ between ordinary light and extraordinary lightis expressed as follows.
In the case of ne＞no,d =(𝑛𝑒2 − 𝑛𝑜2)・𝑡𝑎𝑛𝜃
𝑛𝑒2・𝑡𝑎𝑛2𝜃 + no2・ t

The separation width ‘d’ is the maximum when tanθ= no / ne .
In the case of ne ≒ no, θ=45°, it can be approximated as follows.
d =𝑛𝑒2 − no2・ t
𝑛𝑒2 + no2

For example, in the case of TiO2 (Rutile), ne≒2.69 and no≒2.44 at a wavelength of1550nm. According to the above formula, θmax ≒ 42.2°. So ‘d’ max is calculated to be
0.098 ×t.
θmax ≒ 42.2°, d ≒ 0.098 × t
If θ= 45°, d = 0.097 × t.
That is, the separation width per 1mm of thickness is about 0.1mm.
・Conclusion of the birefringent plate for this device
The separation width does not differ significantly whether the angle of the crystal axis isθmax or 45°. Considering also the reduction of reflected return light, θ is set to about 45°.
Furthermore, we are also good at handling PM fibers. As you know, PMF (polarizationmaintaining fiber) maintains only linear polarization along the slow or fast axis.
There are two know-hows for evaluating high extinction ratio (Fig. 17).
(1) Deterioration of the extinction ratio of PMF depends on wavelength. By inputting
broadband wavelength light into the PMF, wavelength-dependent phase differences canbe evaluated without missing them.
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(2) The extinction ratio can be measured with good repeatability by continuously rotating
the linearly polarized broadband light and inputting it to the PMF, and monitoring theTE and TM power of the output polarized light respectively.
We have a system to handle the PMF with high extinction ratio using these know-hows.
The extinction ratio of the PMF output of commercially available devices is at most 20 dB,
but our devices can achieve ≥ 25 dB or ≥ 30 dB, depending on the class.

Fig. 17: Schematic diagram of the evaluating high extinction ratio of PM fibers
6. Devices for multi-core fiber
As mentioned at the beginning of this paper, we manufacture high-end spatially coupled
optical fiber compact functional devices. In these days, multi-core fiber transmission systems
have been a hot topic in advanced research on high-speed, large-capacity opticalcommunications.
We have fabricated devices for multicore fiber such as FIFOs (Fan-in and Fan-out) (Fig.
18) and multi-functional devices for optical amplifiers using our specialized optical spatial
coupling technology. These devices are characterized above all by low loss and low crosstalk.

Fig. 18: Schematic diagram and photograph of FIFO

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7. Super-continuum Light SourceThis is a highly stable broadband light source using a unique pulse seeder specialized insuper-continuum (SC) generation (Fig. 19). It has a flat spectral shape (Fig. 20) not found inconventional SC light sources and high stable output spectrum (Fig. 21) comparable to ASE
light sources. Due to the dense output spectrum, even passive narrow band-pass filter modules
can be evaluated with high dynamic range. The key technology to achieve flat shape and highspectral stability is in the special fiber laser we developed, and the generation of subnanosecond noise-like pulsed light (Fig. 22) from the fiber laser contributes to them.
This light source consists of single-mode optical fiber, and many optical fiber functional
devices are used. These devises also contribute to reducing the temporal change in output
spectrum.

Fig. 19: Schematic diagram of Super-continuum (SC) Light Source

Fig. 20: Output spectrum

Fig. 21: Time stability of output spectrum-11/12-

8. Conclusion
I introduced several spatially coupled optical fiber functional devices and a SC light source
using them along with their features. I hope that this paper helps you understand the appealof each device, which cannot be understood from only those leaflets.

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