April 24,2025, At the end of the SC177 short course, I asked Professor John Bowers where the upper limit of the modulation rate of semiconductor lasers could be. He is a member of the US National Academy of Sciences, a professor at the University of California, Santa Barbara, and a well-known expert in semiconductor lasers. He should be authoritative on this issue. I remember his answer was that it could reach the Peta level (1000 THz), and the methods to achieve it included different multiplexing technologies such as WDM. I'm not sure if the professor understood my question. Maybe he was answering the upper limit of the optical communication transmission rate. After all, the data of Peta is a bit too large.
For the same question, Doubao gave the following answer:
In the short term (5 to 10 years): It is feasible to commercialize 100 GHz semiconductor lasers through photonic crystal designs, spin lasers, and optimized quantum dot structures.
In the long term (10 to 20 years): With breakthroughs in two-dimensional materials, nonlinear optics, and hybrid integration technologies, THz modulation may become a reality.
I chose Professor Bowers' SC177 short course because I have always been interested in this issue. Just imagine, if the direct modulation rate of commercial semiconductor lasers were not the current 25 GHz but 100 GHz or even 1 THz, then the technologies for realizing 1.6 T or even 3.2 T optical modules would be much easier. So, how does Professor Bowers view this issue?
John Bowers on the class
The problem can be divided into two aspects. First, in terms of direct modulation technology. The Japanese reported at ECOC2019 that based on InGaAsP/SiC materials, with a 6QW Strain and DFB structure, they achieved the highest bandwidth of 108 GHz. The factors affecting the bandwidth of semiconductor lasers include the response frequency (limited by current or power), the damping effect (limited by spectral hole burning or carrier heating), transport (limited by diffusion or tunneling), limitations of parasitic capacitance and resistance, and microwave effects, etc. Taking the improvement of the response frequency as an example, the methods for improvement include reducing the size of the active region and increasing the differential gain (cooling, detuning, size, material, strained QW, doping, threshold gain). NTT's 108 GHz LEAP (Lambda-scale embedded active region photonic-crystal) laser is a representative of high-speed direct modulation lasers.
The advantage of direct modulation lies in its convenience in manufacturing and low cost. However, the disadvantages are the chirp effect and the nonlinear effect. Therefore, Professor Bowers specifically talked about the second aspect, external modulator technology, including lithium niobate MZI modulators, semiconductor MZI modulators, electro-absorption modulators (silicon or III-V), and polymer modulators. Currently, the best modulators on silicon can already achieve a bandwidth of 110 GHz (using lithium niobate, germanium, or the Slow Light technology), and the corresponding transmission rate can be above 224 Gbps. It seems that in the competition with lithium niobate and silicon photonic modulators, EML lasers may be at a slight disadvantage in the future.
From Professor Bowers' introduction, whether it is direct modulation or external modulation, the maximum basic bandwidth is currently around 100 GHz. With the combination of various PAM or coherent technologies, they can all support the next-generation 3.2 T optical transceivers. Do you still remember Broadcom's 200 Gbps VCSEL laser last year? This technology seems to have made further progress at this year's OFC, but it is still only available from Broadcom. To achieve higher-speed modulation of lasers, the optical communication industry still needs to keep working hard.
