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Optical characterization of photopolymer material using λ=532nm and investigation into future use at λ=850nm and λ=1300nm Derek J. Cassidy, Ra'ed Malallah, Min Wan, John J. Healy, John T. Sheridan

Derek J. Cassidy, Ra'ed Malallah, Min Wan, John J. Healy, John T. Sheridan, "Optical characterization of photopolymer material using λ=532nm and investigation into future use at λ=850nm and λ=1300nm," Proc. SPIE 10683, Fiber Lasers and Glass Photonics: Materials through Applications, 106832J (17 May 2018); doi: 10.1117/12.2306163 Event: SPIE Photonics Europe, 2018, Strasbourg, France Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 5/22/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Optical characterization of photopolymer SWW at 532nm and for future use at 850nm and1300nm. Derek J.Cassidy 1, Ra'ed Malallah 1, 2, Min Wan1, John J.Healy 1and John T.Sheridan1 1

School of Electrical and Electronic Engineering, UCD Communications and Optoelectronic Research Centre, University College, Dublin, Belfield, Dublin 4, Ireland. 2

Physics Department, Faculty of Science, University of Basrah, Garmat Ali, Basrah, Iraq.

ABSTRACT The propagation of a light beam through a photo-sensitive photopolymer Polyvinyl Alcohol/Acrylamide (PVA/AA), and the creation of self-written waveguides (SWWs), has received much attention. Here we explore the manufacture and characterization of SWWs in PVA/AA for applications at near infrared communication wavelengths 850nm and 1300nm. The SWWs are fabricated using visible light at wavelength 532nm. The insertion and optical loss of the SWWs at different wavelengths will be interrogated. An optical loss and attenuation profile is to be built up for each of the three wavelengths as they propagate down the resulting SWWs. Keywords: Characterization; self-writing waveguides; photo-sensitized; refractive index; Polyvinyl Alcohol/Acrylamide; multi-mode ; wavelengths; communications;

1. INTRODUCTION Optical networks and systems have always relied on waveguides as a transmission medium for the propagation of light. Optical fibers of 9 µm to 50µm core diameters are the most common waveguide used today. In order to achieve low loss optical connectivity, fusion splicing is used in communication networks, circuits and electronic devices. This is an expensive process that requires specific equipment and is time consuming. Optical fibers are used in communication high bandwidth integrated devices and typically involve the use of micro fiber technology. The development of next generation networks (NGN), and the associated data rate requirements required for the internet of things (IOT), has seen an increase in the use of optical devices. Within this increasingly driven technological environment the need to develop devices that use optical technology and waveguides instead of metallic circuitry is growing. While this approach is

Fiber Lasers and Glass Photonics: Materials through Applications, edited by Stefano Taccheo, Jacob I. Mackenzie, Maurizio Ferrari, Proc. of SPIE Vol. 10683 106832J · © 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2306163 Proc. of SPIE Vol. 10683 106832J-1 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 5/22/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

increasingly finding use in the communication and sensor industry, the interconnection of data communication is making such devices expensive. The development of photosensitized photopolymers, have received much attention in the area of self-written waveguide (SWW) creation [1]. The use of photopolymers is of interest in the field of micro optical engineering. Optical fibers are required to cross small distances, Phloxine B >492nm

(2)

Proc. of SPIE Vol. 10683 106832J-4 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 5/22/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

We introduced 16cm3 of Phloxine B (PB) into the solution. The solution is then heated and stirred for an agreed time (t = 10,800). When the time is reached the heat is turned off and the solution is left to cool after which it is added to storage cuvettes. It is then stored in a dark room for about three weeks so that solution solidifies and is ready for use[3,4].

3.2 Fiber inspection to agreed industry standards, to achieve transmission quality To get accurate optical loss test results and to help us characterisation the MMF 50/50 splitter we must follow the industry standards when it comes to fiber optical cable end face inspection and cleanliness[8]. This is carried out to make sure that no dirt, foreign bodies, scratches or pitting are present on the optical fiber end face, which could skew the results etc. The optical end-face or ferrule (when working with optical connectors) is broken into four different zones and each zone has to conform to an agreed performance relating to the cleanliness of the ferrule [5]. The four zones are as follows; zone A is the optical core of the fibre and the Light transmission path, zone B is the cladding or outer part of the fiber, zone C is the adhesive/epoxy zone and its where the optical fiber meets the ceramic ferrule, zone D is the contact area or ferrule which is the ceramic case for the connector and adds strength to the connection and fiber tip.

A. Core Zone B. Cladding Zone C. Adhesive / Epoxy Zone*

Zones Overlays

D. Contact / Ferrule Zone

A B C

D

Figure 1: The zones of an optical fiber end face

Firstly we must clean the ends of the optical fibers with a Cletop tape cleaner and a one-click bulkhead cleaner. This is a must do exercise to make sure that the optical transmission medium path is clear of any interference[6,7]. To make sure that the fiber end face (FEF) of the fiber is cleaned and that there are no defects an optical inspection scope is used. This


is to identify any issues etc. that could be affecting the transmission of light. The results of cleaning and inspection of the FEF can be seen. Having the fiber cleaned and without any physical abnormalities will allow the testing to commence.

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Figure 2: Example of contaminated and clean optical fiber end face

3.3 The testing and characterization of the Multi-mode Y-Coupler patchlead (MMF 50/50 splitter). To properly test and characterize the MMF 50/50 splitter we must first calibrate the fiber so that we can get an accurate optical loss measurement across[12]. Calibrating a fiber is the term given to the process of measure the optical loss across all the legs of the patchleads and calculating the attenuation coefficient of the optical transmission path. The fact that it’s a 50/50 Y-Splitter will mean that the split coefficient needs to be calculated so that the full optical loss across the MMF 50/50 splitter is known, the overall process of calculating and measuring the optical loss is called the insertion loss test[31]. The process carried out to calibrate the MMF 50/50 splitter is as follows: The Optical light source (OLS) with tunable wavelengths, 1700nm > OLS λ > 500nm, is set up on the bench. We connect up a micro objective lens and align it with the OLS. The FC/UPC connector on Leg 1 of the MMF 50/50 splitter is connected to the mechanized clamp. A Broadband Optical Power Meter (OPM) is then directly connected to the FC/UPC connector of leg 2 of the MMF 50/50 splitter. The OLS is now turned on and the optical power, in watts is set at 1mW, we then align the FC/UPC connector on leg 1 and the micro objective lens so that the optical light intensity of the OLS is directed into the core of the MMF 50/50 splitter. This alignment is achievable by the clamping mechanism allowing for the FC/UPC connector on Leg 1 of the MMF 50/50 splitter and the micro objective lens to couple[8,9,12]. This is setup is to test the optical characteristics of the MMF 50/50 splitter transmission path between leg 1 and leg 2 and across the 50/50 split, which would have a split coefficient of approximately 3.2dBm.


SR = Pi/PT x 100

(3)

IL = -10Log(SR/100) + Гe + 10log

(4)

where IL = splitter insertion loss for the split port, dB Pi = optical output power for single split port, mW PT = total optical power output for all split ports, mW SR = splitting ratio for the split port, % Γe = splitter excess loss (typical range 0.1 to 2 dB),dB These tests are carried out on all four different scenarios as seen in figure 3 below. We also change the wavelength setting on the OLS to the three selected wavelengths 532nm, 850nm and 1300nm and measure the optical loss in the form of mW, which are the calculated as dBm and recorded. This is to test all four optical directions of the light depending on the entry and exit of the light through the MMF 50/50 splitter. This is very important to carry out as it allows for an optical power reference, represented as dBm, for each optical direction. This MMF 50/50 splitter will now be known as a reference fiber with known loss in dBm relative to 1Mw. There are multiple tests carried out in the four directions and the average reference power loss, in dBm, is calculated. These insertion loss tests are very important as they help to attain an insertion loss reference can be attributed to the system for each wavelength[12,31] IL λ (Leg1+2)(dBm) = 10Log[P1(mW)/P2(mW)]

(5)

The optical power for each of the legs can be expressed in the below formula where P1 and P2 relate to the input and output optical power levels of the specific wavelength that is being used to calibrate and characterise the MMF 50/50 splitter. LT={IL(Leg1+2)(dBm) = 10Log[P1(mW)/P2(mW)] +IL(Leg2+1)(dBm) = 10Log[P1(mW)/P2(mW)]}/2

LP = 10.Log [(P1/P2) + 10.Log [(P2/P1)] = dBm

(6)

(7)

Where this is the optical loss across one leg in both directions = insertion loss (IL)


The above formula will allow us to correctly calculate the optical loss across the various legs using optical power settings set at the following values as seen in the attached. These values are used so that the optical loss across the MMF 50/50 splitter on all legs and in each direction can be measured with reference to the transmit optical power level, This result will be the reference loss as measured across the sections under test and represented as dBm[32].


Micro Objective

Fibre Optic MMF Patchlead

OLS λ = 400nm-1700nm

v

v

v

v

FC/UPC Connector Leg 1

OLS = Optical Light Source

Mechanised clamp

FC/UPC Connector

TEST # 1

v

Leg 2

v

OPM λ = 400nm-1700nm

50/50 SPLITTER

FC/UPC Connector

OPM = Optical Power Meter

v

Leg 3

v

v


OPM λ = 400nm-1700nm OPM = Optical Power Meter

Fibre Optic MMF Patchlead FC/UPC Connector Leg 2

TEST # 2

v

v

v

OLS λ = 400nm-1700nm OLS = Optical Light Source

v

Micro Objective

50/50 SPLITTER

Mechanised clamp


v

v


Fibre Optic MMF Patchlead Micro Objective

Leg 2

v

v

v

v


FC/UPC Connector

TEST # 3

50/50 SPLITTER

FC/UPC Connector

Mechanised clamp Leg 3


OPM λ = 400nm-1700nm OPM = Optical Power Meter

v

v


Fibre Optic MMF Patchlead

v

Leg 2

v

FC/UPC Connector OPM λ = 400nm-1700nm

50/50 SPLITTER

TEST # 4 FC/UPC Connector

Micro Objective

Mechanised clamp

v

v

Leg 3

v

OPM = Optical Power Meter



Figure 3: The insertion loss test setup for the MMF 50/50 splitter, showing all four directions.

3.4 The optical loss across the MMF 50/50 splitter using mirror as reflector.

We now disconnect the broadband optical power meter (OPM) from the FC/UPC connector on Leg 2 and before we do anything we give all connectors and optical end faces a good clean and we also inspect them to make sure that there are no contaminants. We now connect up the polished mirror and directly place it in front of the FC/UPC connector on leg 2 of the MMF 50/50 splitter. The end face of the FC/UPC connector and the mirror are directly aligned and in contact with no air gap between them[29].


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Figure 4: The test setup for insertion loss with reflection losses for mirror

The OPM is now connected to the FC/UPC connector of Leg 3 so that the full test circuit is now established and ready for testing with the mirror in place. With everything cleaned and fully connected we then start to carry out the calibration tests with three different wavelengths to determine the optical power level being received by the OPM. The light beam from the OLS is transmitted through the MMF 50/50 splitter. As the light beam passes through the split it undergoes a reduction in power of approximately 3dB. It then continues as it propagates along leg 2 and exits at the FC/UPC connector. It is then reflected of the mirror and transmitted back into the fiber. At this point the optical power of the light beam, at the specific wavelength, undergoes a further reduction in power that is equivalent to 50% or 3dB, as the light passes through the 50/50 split for the second time.. We use the splitter so as to allow for the light to be picked up and measured by the OPM. This is to optically characterize the MMF 50/50 splitter with reference to the input optical power of the OPM and to calculate the insertion loss with reference to the mirror and its reflectance Rλ, with reference to the specific wavelength.

Rλt =ɸr e,(λ0dBm)/ ɸi e,(λi0dBm)

(8)

Where: ɸi e,(λi0dBm) is the spectral power of the specific wavelength received by the mirror. ɸr e,(λ0dBm) is the spectral power of the specific wavelength reflected by the mirror. ILT(Rλt) = {IL(Leg1+2)(dBm) = 10Log[P1(mW)/P2(mW)] +IL(Leg2+3)(dBm) = 10Log[P1(mW)/P2(mW)]}/2

(9) The above formula will give us the insertion loss across the two legs and the split with the MMF 50/50 splitter and the reflectance loss across the mirror[34,35].


3.5

Creating the SWW through the photopolymer.

With the insertion loss now calculated across the MMF 50/50 splitter and the mirror being calculated we now know the reference optical loss or insertion loss for the test setup. We now get the photopolymer (PVA/AA) ready for insertion into the test setup. Under artificial light with approximate wavelength 680nm, we get the prepared PVA/AA from the dark room. By separating the FC/UPC connector from the mirror and cleaning both the mirror and the FC/UPC connector we then place the PVA/AA in between the mirror and the FC/UPC connector, on leg 2. We close the gap so that all three elements; mirror, PVA/AA and FC/UPC connector are in close contact. The testing with the PVA/AA is started. This is where the exposing wavelength of 532nm, incident upon the PVA/AA, will begin the photochemical change process. This will result in the slow creation of a waveguide by the change in the cross sectional area of the refractive index along the axis of wavelength propagation[11]. As the wavelength propagates along the axis of the PVA/AA, polymer chains are created. This is also photo-polymerization is a free-radical polymerization of the polymer. The process to create the SWW will be carried out over an agreed time, where t = 18,000s. This whole process of creating a SWW requires the photopolymer to undergo four distinct steps of polymerization. These steps as listed as Initiation, Propagation, termination and inhibition[3,4,15]. These four steps all form part of the process and follow each other as the SWW is being created, however these will not be discussed in this paper. 3.6 The optical loss across the MMF 50/50 splitter using a mirror as reflector and the photopolymer bulk material in place. When the SWW has been written the process of testing the optical loss or insertion loss across the full setup, including the SWW, will begin[17,18]. All three wavelengths 532nm, 850nm and1300nm will be used to characterize the insertion loss across the SWW[27,28]. With the known reference insertion loss value, in dBm, of the previous tests across the mirror we can we can start the process of characterizing the SWW within the photopolymer. With the previous tests already delivering the reference insertion loss in dBm, this test will allow us to properly calculate and characterize the SWW with reference to the two IR wavelengths used in communications 850nm and 1300nm.

4. SIMULATION AND EXPERIMENTAL RESULTS 4.1 Simulation We set up a simulation of the wavelength 532nm creating a SWW within a photopolymer. The simulation was performed in Mathlab[21]. We also captured the image of the initial stage of SWW evolution with the photopolymer. Another simulation was written to capture a light beam, at wavelength 532nm propagating along the SWW and by reflectance from the mirror, counter-propagating along the same SWW. The SWW became a bi-directional waveguide. To test this


theory we used Mathlab to recreate the photopolymer and with a time exposure texp = 7000 s we used a single 532 nm Gaussian beam with an optical power of 1mW that propagated along the SWW[19,20]. The refractive index change and interaction with the opposite oncoming wavelength being reflected off the mirror can be seen as being represented by a new Gaussian beam with a similar optical power.

500 -

3.5

450 400 -

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1.5

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150 100

0.5 50 -

0 0

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10

15

20

25

30

35

40

45

50

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10

20

30

40

50

Figure 5: The SWW creation across a simulated photopolymer with mirror reflection

4.2 Experimental Results With all the tests carried out and with the final test with the photopolymer the results were collated below in table 1. This table shows the insertion loss for each step as tested and takes into account the splitter losses across the MMF 50/50 splitter. It must be understood that for each direction of light propagation, the optical power of the light beam is halved or reduced by 3dB. All the results take into account the MMF 50/50 splitter section insertion losses and the losses dues to light wave counter propagation on all three wavelengths 532nm, 850nm and 1300nm. nnc [ouuscp OP1scFlns

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Figure 6: The test setup for insertion loss with reflection losses for mirror and across SWW


Source wavelength λ

Insertion loss of Splitter dBm

Total Insertion loss Incl. Mirror dBm

Total Insertion loss Incl. SWW dBm

532 850 1300

-10.34 -9.9 -9.34

-13.65 -12.23 -11.91

-19.97 -25.54 -31.88

Table 1: Results of testing the insertion with mirror and through PVA/AA showing losses across each.

Insertion loss across the different media Total Insertion loss across SWW dBm Total Insertion loss across Mirror dBm Insertion loss across Splitter dBm 0

5

10 1300

850

15

20

25

532

Graph 1: The graph above is a representation of the losses across the different media such as the Coupler, Mirror and Polymer.

5. CONCLUSION When we review the results of the experiments we can see that there is a difference between the three wavelengths as they propagate across the SWW. The wavelengths 850nm and 1300nm have higher losses and their attenuation profile across the test experiment, is consistent with each other compared to the wavelength 532nm. As this wavelength has a higher frequency and is a smaller wavelength, the SWW created through the photopolymer would also be smaller. However the MMF 50/50 splitter patchlead has an optical core of diameter 50µm and the light beam emitted, at wavelength 532nm, would not be as coherent as a single-mode propagating wavelength. As the wavelength 532nm propagated through the photopolymer it can be said that the SWW is attributed to the properties of the incident wavelength. The SWW with the higher cross sectional refractive index, created by the incident light beam has the capability of aiding the propagation of the two higher wavelengths[24,26]. The insertion loss across the SWW for the two communication wavelengths we are testing, 850nm and 1300nm do not allow for the transmission of these wavelengths in normal operating conditions. However under these test conditions we did see the propagation of these two wavelengths across the SWW. This does lead us to believe that the SWWs written by the 532nm wavelength does


have the capability to aid in the propagation of larger wavelengths, even near and infrared wavelengths. With development of infrared photosensitive dyes, the insertion loss across the SWW could be greatly reduced as these larger wavelengths would have a better mode field diameter. This would allow for better propagation across the SWW and less attenuation due to scattering. The use of photopolymers would become a strategic advantage in the area of communications and integrated circuit design. They would be ideal in creating short optical waveguides in integrated circuits and also for the quick jointing and interconnection of optical fibers within the Data Center environment. The capability of the photopolymer to bridge short gaps allows it to someday replace micro-fiber and photonic technology in integrated circuitry. This would aid in the development of faster communication transceivers, reduce costs in manufacture of micro-photonic designs using optical fiber waveguides. More research will need to be undertaken to look at the development of infra-red photosensitive dyes and into the use of the 9µm optical patchlead in the single-mode field of communications. Here we can look into the possibility of creating photopolymers that can aid in the area of multiple wavelength switching through SWWs.

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