FACULTY OF ENGINEERING
LAB SHEET
EOP3046 Optical Waveguide & Devices
Trimester 1 (2013/2014)
OW1 - Experiment on Planar Waveguides
*Note: On-the-spot evaluation may be carried out during or at the end of the experiment.
Students are advised to read through this lab sheet before doing experiment. Your performance,
teamwork effort, and learning attitude will count towards the marks.
EOP3046 Optical Waveguides and Devices
2013/2014
OW1: Experiment on Planar Waveguides
I. Objectives
a) To evaluate the effective index of different planar step-index waveguides by the use of prism
coupling technique.
b) To determine the refractive index values of different planar step-index waveguides.
II. Apparatus
Optical rail bench
He-Ne laser diode with integral drive electronics
Polarizers
Collimating lens
Rotational table
Translation assembly
A few proper mounts
Prism coupling assembly
Planar step-index waveguides
White paper screen.
III. Theory
A planar step-index (SI) waveguide is simply a thin layer of high index material bound on both
sides by material of lower RI (Fig. 1).
Fig. 1.
Schematic diagram of a step-index planar waveguide applied to a suitable substrate.
According to the ray theory, we consider the light propagating along the guide by a series of total
internal reflections (TIRs) that occur at the interface between a material of high refractive index
(RI) and one of low RI provided that the angle of incidence, relative to the interface normal, is
greater than the critical angle. The condition for light guiding in the plane of the waveguide, with
confinement of power in the transverse direction (i.e. perpendicular to the waveguide), is that the
counter propagating components of the field in the transverse direction must interfere to form a
standing wave. This is necessary to ensure that there is no net transfer of power in the transverse
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EOP3046 Optical Waveguides and Devices
2013/2014
direction; otherwise, it will result in a power loss. Applying the standing wave condition to the
transverse field components implies that light can only be guided at certain discrete angles of
propagation; each angle is referred to as a mode. For a planar waveguide of thickness d and RI
n2, the allowed angles, m, are given by the following eigenvalue equation:
2dn2 cos m
(1)
 m  1   3
0
where 0 is the wavelength of the input light, m is an integer (m = 0, 1, 2,…) called the mode
numbers, and the terms 1 and 3 are the evanescent field phase shifts at the waveguide
boundaries with the surrounding material.
Each mode propagates with a particular phase velocity characterized by a mode effective
index, ne (where ne = n2 sinm). Eq. (1) can be re-written in terms of the mode effective index as

2d n22  ne2

1/ 2
0
 m  1   3
(2)
where 1 and 3 are given by
 ne2  ni2 

(3)
 i  tan   2
2 
n

n
e 
 2
with ni (i = 1, 3 as shown in the Fig. 1) as the RIs of the surrounding materials, and  depends on
the polarization state of the guided light ( = 1 for TE modes and  = n22 / ni2 for TM modes).
1
Light may be coupled from a collimated or focused beam into a waveguide using a high RI
prism clamped to the waveguide surface (Fig. 2). The conditions for coupling to take place are as
follows:
 The prism RI np must be greater than that of the waveguide material.
 The prism and the waveguide must be in good optical contact such that the evanescent
field generated by TIR at the bottom surface of the prism penetrates into the waveguide
(i.e. the contacting surfaces must be clean and dust free).
 The effective index (and hence the phase velocity) of the light wave along the bottom
surface of the prism must be adjusted to match that of a waveguide mode. This is
achieved by adjusting the angle of incidence at the input face of the prism by rotation of
the prism coupling assembly.
Fig. 2.
The prism coupling arrangement.
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EOP3046 Optical Waveguides and Devices
2013/2014
With reference to Fig. 2, the effective index, nep, of the advancing field along the prism
surface in contact with the waveguide is nep = np sinp, where p = A + t (A and t, being the
prism angle and the transmitted angle at the prism input face, respectively). Snell’s law enables
t to be expressed as np sint = sini. This means that the mode effective indices, ne, of all the
waveguide modes, to which coupling is observed, may be found from the following expression:

 sin  i (m) 


(4)
ne  nep  n p sin  A  sin 1 
 n



p



where i(m) is the measured angle of incidence, relative to the normal to the prism input surface,
which results in excitation of the waveguide modes of number m. Usually, following the sign
convention, incident angles which extend clockwise from the normal are taken as positive and
those which extend anti-clockwise are taken as negative.
Light within a guided mode may be coupled back out of the waveguide by clamping a second
(output) prism to interact with the mode at a distance of about 1 – 3 cm from the input prism
(Fig. 3b). If the output prism is the same as the input prism, then the light coupled out of the
guide will emerge at the same angle, relative to the output face normal, as the input angle i(m).
Most of the power emerges in a beam, which is in the incident plane. When the emergent light is
viewed on a white screen, an illuminated line with a bright spot at its center is seen. The line,
referred to as a mode line or m line, is in fact a shallow arc resulting from the intersection of the
emergent cone of light with the screen.
Fig. 3.
The prism coupling assembly; (a) side view, (b) top view.
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EOP3046 Optical Waveguides and Devices
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IV. Experimental Procedures
a) Set up the optical rail according to Fig. 3 with the beam from the semiconductor laser
intersecting the axis of the rotational mount. By using the optical lens, focus the beam onto
the rear plane of the prism which can be rotated to vary the angle of incidence. The prism
coupling assembly should be fixed to the rotational stage in the orientation as shown in Fig.
3b. The laser orientation may be adjusted such that the beam propagates along the optical
axis. The polarizer and lens should be mounted with their centers on the optical axis, and the
laser beam focused onto the rear face of the coupling prism close to the right angle. The focal
length of the lens is approximately 15 cm. The prism to lens distance may need to be altered
to ensure that the beam diameter is indeed at a minimum in the region of the rear surface of
the prism. This can be checked by viewing the beam on a white paper/screen. The incidence
angle of the laser beam onto the prism input surface may be varied simply by rotating the
prism table. The rotational stage is fitted with a Vernier scale.
b) Adjust the polarizer to achieve the TE polarized state of the incident beam. The polarization
direction of the laser output may be defined by firstly fixing the transmission axis of the
polarizer at the desired angle, and then rotating the laser about its axis to achieve maximum
transmission (i.e. maximum brightness of the illuminated spot when the beam intersects the
white screen). The more accurate approach is to fix the transmission axis of the polarizer at
900 to the desired polarization direction, and to adjust the orientation of the laser for
minimum transmission as viewed on the screen intersecting the beam. The polarizer must
then be adjusted through 900 to obtain the required polarization state at optimum power.
Firstly, the polarization state of the incident light should be arranged to be vertical to launch
TE modes, using one of the techniques above. Clean the bottom surface of the prism well.
The quartz slide supporting the waveguide should then be clamped firmly into the prism
coupling assembly as shown in Fig. 3b, but without output prism (note the viscosity reading
marked on the slide to be Medium or High). In order to achieve prism coupling to either the
substrate modes or the guided modes, the prism to waveguide contact must be extremely
good since the evanescent field penetration depth is only about 1 m. This means that the
prism surface must be dust free. The optimum coupling is best achieved at the point of
closest contact, where the screw clamp applies the greatest pressure to the slide. In order to
establish and optimize the coupling, the horizontal position of the prism/waveguide assembly
must be finely adjusted using the micrometer on the translation stage. When this is achieved,
a series of bright spots will appear on the front and rear faces of slide. The bright spots
appear at the points where the coupled beam undergoes TIR at the slide surfaces.
c) Following observation of substrate modes, rotate the prism coupling assembly in an anticlockwise direction until the coupling to a guided mode of the planar waveguide, which is
characterized by a continuous illuminated line, is observed. As the prism stage is rotated, the
coupling to the substrate modes may become weaker as the optimum coupling position is
lost.
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EOP3046 Optical Waveguides and Devices
2013/2014
To observe the m lines, clamp the second prism to your waveguide to interact with the guided
mode at a distance of about 1 – 3 cm from the input prism. If the optical contact is
sufficiently good, light will couple out of the guide, and the m lines may be observed on a
white paper held about 20 cm from the prism. Rotate the prism assembly and note what
happens to the m lines as power couples selectively to each successive mode.
d) Measure the angles i(m) relative to the normal to the prism input facet, at which coupling is
observed to each of the modes of the waveguide. The orientation of the rotational stage
relative to the prism is arbitrary, and the reference angle at which the incident beam is normal
to the prism face can be found by rotating the prism assembly until the beam reflected from
the front face of the prism is in the same vertical plane as the incident beam. Use a white
paper placed above or below the laser output in order to be assisted in this task. The coupling
angles i(m) are simply the absolute difference between the scale readings at which the
coupling is observed and the scale reading for the reference angle, for which the sign
convention used in eq. (4) and the experimental set up of fig. 3b dictates that angles
extending clockwise from the prism normal are positive, and those extending anti-clockwise
are negative.
e) Once all of the coupling angles are measured, calculate the corresponding mode effective
indices using eq. (4), and draw up a table of mode numbers m, their measured coupling
angles and their effective indices ne. Here np is the prism refractive index which is 1.7789,
and the prism angle A is 600.
f) Repeat the above experiment for TM polarization state.
Observation Table I
Viscosity
Mode
Number
(m)
High
0
High
High
Medium
1
2
0
Medium
1
Medium
2
TE Modes
i
ne
TM Modes
i
ne
g) On the basis of the above table I, the waveguide RI n2 can also be obtained. In eq. (2) all of
the parameters are known except the waveguide index n2 and the thickness d. 0 is the input
wavelength (633 nm), n1 (= 1) is substrate (air) RI, and n3 is the RI of the quartz substrate (=
1.456). By substituting the measured values of the effective indices, and their corresponding
mode numbers into eq. (2) for two of the modes, two simultaneous equations with two
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EOP3046 Optical Waveguides and Devices
2013/2014
unknowns can be established. It is difficult to solve for n2 and d analytically, since n2 appears
on both sides of the equation, and a numerical approach must be adopted. The simplest way
is to plot d as calculated using eq. (1) against n2 for the m = 0 and m = 1 modes over the
appropriate range. The true values of d and n2 may then be read from the crossing point of
the two curves. For this exercise, one should use the correct value of n2. It is known that
value of the effective index of the m = 0 mode of a multimode waveguide is a little less than
the waveguide material RI. Hence for the calculation of d vs. n2, one should choose an initial
value of n2 (which is say 0.002 above the measured effective index of the m = 0 mode) and
then increment by 0.002 for calculating d each time. This should be continued until the d vs.
n2 curves for the m = 0 mode and the m = 1 mode cross.
Using the approach described above, determine n2 and d using the m = 0 and m = 1 modes for
the TE polarization, and repeat the exercise using the TM mode data. Repeat this exercise for
the second waveguide.
Analyze the data gathered in the observation table I and discuss your analysis in the report.
V. Precautions
a) All the equipments/components are extremely sensitive to the conditions as well orientations.
So, please handle with great care. Keep the components away from dust as it would greatly
affect the experiment, and the results may not be obtained that easily.
b) Never put your eyes directly in front of the laser beam. It is advisable to put the rings or wrist
watches off whenever one works with lasers.
VI. Lab report format, Submission and Evaluation
a)
An individual report should be nicely typed and should include the following:

Date of experiment

Names and IDs of all members

Objectives

Apparatus

Summary of procedures/designs

Results/answers for all the assignments/questions

Analysis and discussions
In this section of the report, you are expected to analyze the results obtained in the
experiment. In your analysis, you are expected to analyze the reason for obtaining
such results. One may link from the theory of waveguiding that discusses the
condition of waveguiding, modes in the waveguide and the respective transverse
modes.

Evaluation
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EOP3046 Optical Waveguides and Devices
2013/2014
In this section of the report, evaluate the expected results of the same experiment if a
laser at 550 mn was used instead of the He-Ne laser. Provide the analysis of the
expected results in your discussion.


Conclusions
References
b) Duration of lab report submission must not be more than 7 days after the date of your
experiment.
c) Lab report must be submitted to the lab staff of Optical laboratory.
d) Marks will be deducted for late submission or plagiarized report.
Marking Scheme of lab sheet
On the spot evaluation: 2.5 marks
Experiment: 2.5 marks
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