Coupling, Collimation & Termination
Wei-Chih Wang Institute of Nanoengineeirng and Microsystems National Tsing Hua University
w. wang 1 Week 14
• Course Website: http://courses.washington.edu/me557/sensors • Reading Materials: - Week 14 reading materials can be found: http://courses.washington.edu/me557/reading/ • Proposals need to be sent to me to get the credits (Only two so far) • Work on Lab 2 (arrange time to meet with TA, please finish it this week) • HW 3 due today • HW 4 assigned due week 16 if need more time send the HW electronically to [email protected] after week 16 • Final presentation is on 12/23, final report due 1/7/20
W. Wang 2 Outline
• Guided modes in symmetric and asymmetric slab waveguides • General formalisms for step-index planar waveguides • Multilayer structure • Analytical approach to various rectangular waveguide • Examples W. Wang 3 Outline
• Waveguide collimation and termination • Losses and dispersion in waveguides
W. Wang 4 Next Week
• Intensity based sensor • Phase modulation sensor • Periodic structure sensor
w.wang 5 Fiber Direct Focusing
Bare fiber coupling
fiber X-Y lens stage w. wang 6 Pigtailed and connectorized fiber optic devices
w. wang 7 Mechanical Splicing
w. wang Bare Fiber to Fiber Connection 8 Mechanical coupler
SMA Fiber Optic Coupler
w. wang 9 Fiber connector types
Biconic Connector A single fiber connector, body has a cone shaped tip, and a threaded barrel for securing to the coupler. Ferrule can be either ceramic or stainless steel. Generally heat cured. Mainly found on older electronic equipment and infrastructure. Generally considered a high loss connector.
w. wang 10 ST Connector A single fiber connector with either composite or ceramic bayonet style ferrules (2.5mm). Connector body is molded plastic using a twist- lock latching mechanism. This style of connector is found in many applications, one of the first truly universal connector. Also used in APC (angled) applications.
w. wang 11 FC Connector A single fiber connector with a standard (2.5mm) ceramic ferrule. Connector body can be metal and or plastic molded, and the threaded keyed barrel ensures reliable coupling. This is a good style for high vibration environments. Also a popular APC (angled) style. Found in telecommunication equipment and CCTV & CATV applications.
w. wang 12 Interchangeable connector
Hybrid SC connector adaptors are available for: MU, FC, SC, ST, LSA ( DIN47256 ), F3000, E2000, LC optical connector types.
Kingfish international w. wang 13 Waveguide Coupling
p type waveguide
light emitting layer
n type incident Direct Focusing heat sink beam substrate thin-film End-Buttlaser diode Coupling
incident o reflected beam beam
air n surface wave 1
film waveguide n2
substrate n3
Grating Coupler Prism Coupler w. wang 14 Fiber/waveguide to Waveguide Coupling optical fiber clips waveguide silicon nitride clips
silicon
V-groove with clip connection
Tapered Mode Size Converters w. wang 15 w. wang 16 =
m
w. wang 17 =
w. wang 18 normalization reflection Area mismatch Overlap integral
w. wang 19 Interference created By waveguide and Fiber surfaces
X= misalignment w. wang 20 End-butting Method
1. practical in case of coupling a waveguide to a semiconductor laser or to another waveguide such as the end of a commercial optical fiber. 2. Efficiently coupling an uncollimated divergent laser beam (10 to 20o) emitted from a semiconductor laser, which is difficult to achieved using either prism, grating, or tapered film couplers.
w. wang 21 High efficient coupling is achieved by making the thickness of the waveguide approximately equal to that of the light emitting layer of the laser and aligned as shown in Figure 13a (A.Yariv, IEEE J. QE-9, 919 (1973)), and also by the fact that the field distribution of the fundamental lasing mode is matched to the
TE0 waveguide mode. The efficiency can be further improve if indices of the laser emitting layer and waveguide are close and the ratio of the thickness of waveguide to the laser emitting layer is small. To eliminate any oscillatory shape of output due to the Fabry-Perot etalon formed by the plane parallel faces of the laser and waveguide (when separation between them are less than a wavelength), epoxy of the matching index between the laser and waveguide must be used. This method is useful if an unpackaged laser diode is used.
w. wang 22 Optical Analysis
• Find Modes
• Find Maximum Coupling Efficiency
• Total Power Out
R. Panergo, W. Wang, “Resonant Polymeric Waveguide Cantilever Integrated for Optical Scanning,” Journal of Lightwave Technology ( Volume: 25, Issue: 3, March 2007 )
w. wang 23 SU-8 optical scanner
cantilever waveguide U groove tapered waveguide
R. Panergo, W. Wang, “Resonant Polymeric Waveguide Cantilever Integrated for Optical Scanning,” Journal of Lightwave Technology ( Volume: 25, Issue: 3, March 2007 )
w. wang 24 Mode Coupling (MC) • Divided into 3 sections – Fiber input to facet of the waveguide – Taper section – Interface between taper to the beam Y
Z
Taper Fiber Input Beam
w. wang 25 Optical Parameters
• Index of Refraction
–nsu8 = 1.596, nSiOx = 1.46, nair = 1
• Input Source/tapered Fiber (Dcore=62.5m) – Single Mode, 633nm wavelength • Film Thickness – Thickness 100 micron – Initial Width 100 micron – Final Width 50 micron
w. wang 26 MC – Fiber to Waveguide • Initial Assumptions – Input is a single mode Gaussian beam (end butt coupled) – Ignore loss due to scattering and absorption • SU-8 Waveguide with 85x230m cross section • 633nm light source through a 62.5m core fiber
Cross Section Top View
Air Air
SU-8 SU-8
X Y SiO2 Air
w. wang 27 Fiber to Waveguide continued … • Coupling efficiency determined by overlap integral:
2 A(, x y ) B* (, x y ) dxdy m m A(,x y )22 dxdy B (, x y ) dxdy m
• A(x,y): Amplitude distribution of input source
• Bm(x,y): Amplitude distribution of the mth w. wangmode 28 Fiber to Waveguide continued … • The first 100 combinations of modes Mode Coupling Eff. (%)
were examined TE0,0 61.44
TE0,2 29.66 •TE0,0, TE0,2, and TE0,4 TE 6.92 couple 98% of the light 0,4
• All 100 combinations couple 99% of the light
• Assume that 100% coupling w. wang 29 Fiber to Waveguide continued … • Consider mis-alignment effect
w. wang 30 MC – Taper Section • Photolithography process produced step-like features • Mask for process was printed with a high resolution printer – Resolution: 2450 dpi horizontal, 300 dpi vertical
w. wang 31 Taper Section continued … I II III Z ∆Y
Y B
A
2 A (y)B* (y)dy n m ηtaper A(y)d22y B(y)dy nm
w. wang 32 Taper Section continued …
ηtaper Y
∆Y Z
wf wi Nstep
Nstep wwi f Nstep ttaper taper 2y w. wang 33 Taper Section continued …
SU-8 Δy= 6.5m
ttaper = 96%
w. wang 34 MC - Taper to Beam
Output from Taper Input to Beam • Index change from taper section to beam Air Air SU-8 SU-8 2 AxBxdx()* () nm X SiO X Air beam 2 Ax()22 dxB () x dx nm Output from Taper Input to Beam
X Air • Y direction remains Air unchanged SU-8 SU-8 Z SiO2 Air • loss is <<1% and assumed to be w. wangnegligible 35
MC – Total Coupling
i ttaper beam Y
Z
Fiber Input Taper Beam
ittaperbeam
w. wang 36 Prism Coupler
Check out the derivation on w. wang evanescent waver Recall
or phase matching condition
w wang 38 w wang 39 Evanescent wave
use for sensing and wave coupling w wang 40 Prism Coupler
Key is making sure both prism modes and propagating modes exist aso we can generate evanescent wave for coupling!
w. wang 41 w. wang 42 w. wang 43 w. wang 44 w. wang 45 Why prism coupler?
The advantage of prism coupler is that it can be use as an input and output coupling devices.
If more than one mode is propagating in the guide, light is coupled in and out at specific angles corresponding to each mode.
If a gas laser is used, the best method for coupling is using either prism or grating coupler
Disadvantage is is that mechanical pressure must be applied to prism during each measurement sot hat spacing between prism and waveguide remains constant to get consistent coupling coefficient.
Other disadvantage is prism coupler index must be greater than the waveguide.
Another disadvantage is that the incident beam must be highly collimated because of the angular dependence of the coupling efficiency on the lasing mode. w. wang 46 w. wang 47 w. wang 48 Assignment
What are the coupling angles for modes that are guided? Given n1 = 1.0, n2= 1.6, n3=1.5, np =2.2 and waveguide thickness h = 10m, = 1.310um and W= 9 m and base of the prism is 1mm.
w. wang 49 Grating Coupler See if coupling in, there is a reflection and possible substrat mode coupling.
Gold (200nm) ITO (200nm) Input Port (Incident Waveguide(400nm) Beam) Gold (200nm)
The light coupled into the thin film is achieved by the fact that the diffracted incident light is phase-matched to a mode of the film. Grating couplers viewed as surface-wave to leaky-wave converter (output coupler)
w. wang 50 Because of its periodic nature, the grating perturbs the waveguide modes in the region underneath the grating, thus causing each one of them to have a set of spatial harmonics with z-direction propagation constants given by
m m m Diffraction grating The fundamental factor is approximately equal to the of the particular mode in the waveguide region not covered by the grating. Because of the negative values of m, the phase matching condition m=kn1sinm (continuity of tangential field component) can now be satisfied so that
m
w. wang 51 z
x
nˆ (E1 E2 ) 0
w wang 52 We just solve the same two laws we know about geometrical optics using phase matching and BC in wave equation: 1) the laws of reflection 2) The law of refraction.
n1 sin1 n2 sin2 i r
x z
kx k sini krx kr sinr ktx kt sint w wang 53 w wang 54 Why grating coupler?
1. A simple reproducible and permanent coupler compatible with planar device technology.
2. The grating coupler can also be used on high-index semiconductor waveguide where it is difficult to obtain suitable prism material.
w. wang 55 Example
Grating: = 0.4m on a GaAs planar waveguide
o = 1.15m
Propagation constant for the lowest-order mode in the waveguide:
o=3.6k
Assume 1st _order coupling, || = 1, what incident angle should the Light make in order to coupe to the lowest-order mode?
w. wang 56 Assignment
Grating: = 0.4m on a SiO planar waveguide
o = 1.310m
Propagation constant for the lowest-order mode in the waveguide:
o=3.6k
Assume 1st _order coupling, || = 1, what incident angle should the Light make in order to coupe to the lowest-order mode?
At what 0 do we start to need higher-order coupling?
w. wang 57 w. wang 58 Fiber to Fiber coupling loss
Joseph C. Palais
w. wang 59 Fiber to Fiber connection loss
• Reflection losses • Fiber separation • Lateral misalignment • Angular misalignment • Core and cladding diameter mismatch • Numerical aperture (NA) mismatch • Refractive index profile difference • Poor fiber end preparation w. wang 60 Fiber mismatches
w. wang 61 Fiber to fiber connection loss
The loss in optical power through a connection is defined similarly to that of signal attenuation through a fiber. Optical loss is also a log relationship. The loss in optical power through a connection is defined as:
w. wang 62 Fiber to fiber connection loss
Intrinsic coupling losses are limited by reducing fiber mismatches between the connected fibers. This is done by procuring only fibers that meet stringent geometrical and optical specifications. Extrinsic coupling losses are limited by following proper connection procedures.
w. wang 63 GRIN Lens
Gradient index micro lenses represent an innovative alternative to conventional spherical lenses since the lens performance depends on a continuous change of the refractive index within the lens material.
1. GRIN objective lenses with an angle of view of 60° are produced in standard diameters of 0.5, 1.0 und 1.8 mm. Typical object distances are between 5 mm and infinity.
2. Instead of curved shaped surfaces only plane optical surfaces are used which facilitate assembly. The light rays are continuously bent within the lens until finally w. wang 64 they are focused on a spot. GRIN Lenses
2 n(r) = no(1-Ar /2)
Where
n0 -- Index of Refraction at the Center r r -- Diameter of Grin Lens A -- Gradient Constant P The quadratic n(r) results in a sinusoidal ray path
P = 2/A0.5 For length L = P/4 => quarter pitch lens = P/2 => ½ pitch lens w. wang 65 0.23P 0.25P 0.29P
w. wang 0.5P 0.5P 66 GRIN Lens
Light exiting a fiber can be collimated into a parallel beam when the output end of the fiber is connected to the GRIN lens. (0.25P)
w. wang 67 GRIN Lens Focusing of the fiber output onto a small detector or focusing of the output of a source onto the core of a fiber can be accomplishing by increasing the length of the GRIN lens to 0.29 pitch. Then the source can be moved back from the lens and the transmitted light can be refocused at some point beyond the lens. Such an arrangement is useful for coupling sources to fibers and fibers to detectors.
w. wang 68 w. wang 69 Coupling between Waveguides
2x2 Coupler
Thor Labs w. wang 70 w. wang 71 w. wang 72 w. wang 73 1 mm
1
2.1
w. wang 74 w. wang 75 w. wang 76 w. wang 77 w. wang 78 w. wang 79 w. wang 80 Fusion splicer
Fusion splicing is the act of joining two optical fibers end-to-end using heat. The goal is to fuse the two fibers together in such a way that light passing through the fibers is not scattered or reflected back by the splice, and so that the splice and the region surrounding it are almost as strong as the virgin fiber itself. The source of heat is usually an electric arc, but can also be a laser, or a gas flame, or a tungsten filament through which current is passed.
w. wang 81