Kramers Kronig Dispersion Relations and MZ Modulator

Wavelength detuning in MZ modulator is important parameter as the change in Index of Refraction depend on absorption coefficient change alpha. The larger detuning leads to larger loss due to alpha change from Kramers Kronig Relation. If the absorption peak is situated further away from operating wavelength, the change of refractive index is small and hence need larger voltage for sufficient pi phase shift. We can use absorption measurements and Reflectance measurements to calculate the real part of refractive index and the phase. this will enable us to find optimum parameters for the devices.Refractive Index change is highly polarised due to its dependence on change in Alpha which is the transition between electron state E1 and heavy holes state HH1 tends to be TE polarised. The change in refractive index is due both QCSE and Pockels effect even though Pockels effect is in the order of magnitude smaller compared to QCSE effect nonetheless not negligible when the detuning is much larger than 160nm.

Electro-absorption vs Mach-Zehnder Modulators

Probably, i have already talked about modulators, as the speed and bandwidth drives the technology ever forward i will just introduce two types of modulators used in optic networks. Electro-absorption modulators exploit the properties of materials under the influence of electric field to modulate the light carrying signals by absorption.
This type of modulators have a drawback for having a limit to how fast you can modulate light due to material properties especially the incumbent InGaAsP where valence band offset limit the depletion of holes.
Mach-Zehnder modulators uses intensity modulation where by a pi phase change between the path length leads to modulation of light. We can also use the QCSE effect to improve the Mach-Zehnder by reducing Voltage needed to change phase to pi we can use Kramers-Kronig relation to calculate the effect of absorption coefficient change on refractive index.

Back to Uni with tales from Caswell

As i pack my belongings for my journey back to University after 3 Months of placement at work i feel overcomed by sheer joy of achieving my goals at Caswell.

I know I will miss cycling back and forth to work through beautiful countryside scenery, I believe the images of farms are vividly emblazoned in my mind. I bade good bye to my adopted team of beautiful people and head for the train station at Milton Keynes. The birds who without fail woke me up early in the morning with their sweetest songs will be servely missed and whiff of manure that permiated the air as i cycle through narrow roads on the hills will be remembered always.

the hope that experience gained at Oclaro is weighs heavily in my mind on how to translate my knowledge into sustainable means of income.

I am glad i saw the other side of real world application of my academic knowledge.

Now i am looking forward to wrap up and finish my studies (GW). Surrey here i come......

InGaAsP versus InGaAlAs

Most of semiconductor devices used in optical communication networks are made of InGaAsP material. It has since became obvious that devices made of InGaAsP have lower performance due to high thermal sensitivies of this material. Huge effort to find material that will replace the incumbent InGaAsP has been deployed.

Thermal sensitivity of InGaAsP is written in its band structure. It has smaller conduction band-offset allowing thermally activated carriers to escape easily into barriers, and larger valence band offset inhibiting quick carrier emptying. There are many suitable candidates one of them being InGaAlAs which have larger conduction band offset and smaller valence band offset compared to InGaAsP. The race to depose the incumbent InGaAsP is on with new exotic materials such as dilute Nitrides, Bismides and many more...

A Trip to Natural History Museum in New York

As the Snow mountains melt in New York, i headed for the Natural History Museum from Columbus Avenue, just as stone throw to the museum. Large crowd amassed outside ready to get in..People from different nations and ages gathered outside in the morning chill from very cold breeze blowing from the Central Park across the road.

"How Much is the fee to get in", i asked the lady in uniform at the reception desk.

"Well the entry is by donation you can pay any amount but preferred amount is this much for adults and this much for kids".

I pulled out 10 bucks and she asked "how much you want pay", I felt that was enough for everything i was about to see in this vast building with vast collection of different artefacts..the hours came and went while i was gazing at amazing exhibits and never felt the urge to go back to the Broadway Street.

Photocurrent Measurement of Quantum Well Device

We can obtain information about the device structure using photocurrent measurements. As the word photocurrent suggest the measurement involves the use of light to excite electrons for valence band to conduction band by absorption of photons by the electrons. As absorption in semiconductor is determined by the density of photons, density of electrons in valence band and density of empty states in conduction band. if the incident photon has energy larger than bandgap, electron absorbing it will be excited to the conduction band and this is current generation and can be detected using very sensitive equipment such as lock-in amplifier in my case SR830. if we run negative bias across the device the amount of photons with lower energies absorbed increases. Therefore we can determine the bandedge and hence use this feature in Modulators. It is interesting to observe an absorption peak just below the bandedge due to the presence of excitons. The presence of excitons is important feature for Modulators exploiting Quantum Confined Stark Effect.

Finally I get what i was looking for

Photocurrent Measurement can be seen as easiest of experiments to do..i beg to differ especially when dealing with lots of unknowns such as devices which have defects. You will spend days probing but you won't get what you are looking for...But perserverance pays big time.. i have never seen beautiful spectrum like the one i saw today.. I can see excitonic states at work, excitation between energy levels in quantum well as well as the barrier materials...so today is exciting day for me...i can see electrons in the valence band gobbling up incoming photons and get very excited such that they jump over the band gap (that is absorption). while sending in photons of increasing energy (from higher wavelength (low energy) to lower wavelength (higher energy)), electrons get busy absorbing incoming photons and jump to create photo-induced current.