Self-op­ti­mi­zing laser diode sys­tems

In diode la­sers, the gain me­di­um is rea­li­zed as a se­mi­con­duc­tor com­pound with a di­rect band gap. In con­trast to solid state laser sys­tems like the Ti:sapp­hi­re laser, this enables di­rect elec­tri­cal pum­ping of the ac­tive zone. The pro­cess of elec­tri­cal pum­ping sti­mu­la­tes elec­trons from the va­lence band into the con­duc­tion band from were they can relax back into the ground state by the emis­si­on of pho­tons, with an en­er­gy equi­va­lent to the band gap en­er­gy. This me­cha­nism re­veals ano­ther ad­van­ta­ge of diode la­sers: by al­te­ring the com­po­si­ti­on of the ac­tive me­di­um, the band gap en­er­gy and the­re­by the emis­si­on wa­ve­length of the de­vice can be chan­ged. The so cal­led “band gap en­gi­nee­ring” is a well es­ta­blis­hed tech­ni­que in the se­mi­con­duc­tor in­dus­try and al­lows for a huge co­ver­a­ge of dif­fe­rent wa­ve­lengths with laser di­odes. This spec­tral fle­xi­bi­li­ty is an ad­van­ta­ge com­pa­red to solid state and fibre la­sers.

While in some areas, like tu­ne­able light sour­ces with nar­row line width, laser di­odes are sta­te-of-the-art, laser diode sys­tems are not com­mer­ci­al­ly avail­able as a light sour­ce for ultra short pul­ses. Theo­re­ti­cal­ly, the lower bo­un­da­ry for pulse widths ge­ne­ra­ted by laser di­odes should be below 50 fs. The pul­ses are ge­ne­ra­ted by lo­cking the lon­gi­tu­di­nal modes of the laser ca­vi­ty. The lo­cking me­cha­nism is rea­li­zed by a sa­tura­ble ab­sor­ber. For this re­a­son it is cal­led pas­si­ve mode lo­cking. In the case of a laser diode a pro­blem ari­ses: the strong cou­pling of the real and ima­gina­ry part of the sus­cep­ti­bi­li­ty leads to a strong wa­ve­length chirp. This leads to a tem­po­ral broa­de­ning of the pulse and a re­duc­tion of the spec­tral band­width. This me­cha­nism in­hi­bits the di­rect ge­ne­ra­ti­on of sub-50 fs pul­ses.

A first step to solve this pro­blem was the in­tro­duc­tion of in­tra­ca­vi­ty dis­per­si­on ma­nage­ment (IDM) by using the Fou­rier-ex­ter­nal ca­vi­ty laser (FTE­CAL) setup. The FTE­CAL setup is de­pic­ted in pic­tu­re 1. A two sec­tion laser diode with an an­ti-re­flec­tion coa­ting emits light which is spec­tral­ly dif­frac­ted by a gra­ting. The first dif­frac­tion order is colli­ma­ted by a lens while its spec­tral com­po­n­ents are fo­cu­sed. A mir­ror in the Fou­rier plane re­flects the light back into the laser diode. The zeroth order is used for out­put cou­pling. The setup is si­mi­lar to a fol­ded gra­ting com­pres­sor, which is used for pulse com­pres­si­on: by mo­ving the gra­ting out of the focus li­ne­ar chirp is ge­ne­ra­ted. With a sui­ta­ble ali­gnment we were able to ge­ne­ra­te 200 fs pul­ses with the usage of an ex­ter­nal pulse com­pres­sor. Howe­ver, mo­ving the gra­ting out of the focus will al­ways in­tro­du­ce a slight spa­ti­al chirp, which has a ne­ga­ti­ve in­flu­ence of the pulse ge­ne­ra­ti­on.

© RUB/ETIT/PTT

Fig. 1: FTECAL setup with double mask SLM in the Fourier plane

In order to avoid this pro­blem we are try­ing in the scope of a pro­ject (fun­ded by the Deut­sche For­schungs­ge­mein­schaft) to com­pen­sa­te the wa­ve­length chirp with a spa­ti­al light mo­du­la­tor (SLM). This has the ad­van­ta­ge com­pa­red to the pre­vious ap­proach, that we are able to com­pen­sa­te ar­bi­tra­ry chirp while avo­iding pro­blems due to spa­ti­al chirp. In ad­di­ti­on, the SLM al­lows the ma­ni­pu­la­ti­on of the spec­tral am­pli­tu­de. The SLM is con­trol­led by an evo­lu­tio­na­ry al­go­rithm which op­ti­mi­zes the laser sys­tems on pa­ra­me­ters like shor­test pulse width and hig­hest spec­tral band­width. We aim to ge­ne­ra­te sub-100 fs pul­ses with this ap­proach.

Ano­ther field of ap­p­li­ca­ti­on is the use of the sys­tem in a tera­hertz (THz) time do­main spec­tro­me­ter, which can be used for spec­trosco­pic ap­p­li­ca­ti­ons in the re­gi­on from 100 GHz up to se­ver­al THz or for ma­te­ri­al tes­ting. The most im­portant fea­ture here is the pulse width, be­cau­se it is di­rect­ly pro­por­tio­nal to the band­width of the THz sys­tem. The use of a laser diode sys­tem has the po­ten­ti­al to si­gni­fi­cant­ly re­du­ce the costs for THz sys­tems. In ad­di­ti­on, the shaping of the op­ti­cal spec­trum could lead to high­ligh­ting of selec­ted areas of the THz spec­trum. This would lead to a hig­her fle­xi­bi­li­ty in con­trast to com­mer­ci­al laser sys­tems.

Re­fe­rence:

  • [1] M. A. All­oush, R. H. Pilny, C. Bren­ner, A. Klehr, A. Knig­ge, G. Tränk­le, and M. R. Hof­mann, “Pas­si­ve, ac­tive, and hy­brid mo­de-lo­cking in a self-op­ti­mi­zed ul­tra­f­ast diode lase”, SPIE 10553, Novel In-Pla­ne Se­mi­con­duc­tor La­sers XVII, 105530N (2018)
  • [2] R. H. Pilny, B. Döpke, J. C. Bal­zer, C. Bren­ner, A. Klehr, A. Knig­ge, G. Tränk­le, and M. R. Hof­mann, “Fem­to­se­cond se­mi­con­duc­tor laser sys­tem with re­so­na­tor-in­ter­nal dis­per­si­on ad­ap­ta­ti­on”, Op­tics Let­ters, Vo­lu-me 42, Issue 8, pp. 1524-1527 (2017)
  • [3] R. H. Pilny, B. Döpke, J. C. Bal­zer, C. Bren­ner, A. Klehr, G. Er­bert, G. Tränk­le, and M. R. Hof­mann, “In­ter­ac­tion of phase and am­pli­tu­de shaping in an ex­ter­nal ca­vi­ty se­mi­con­duc­tor laser” - SPIE 9767, Novel In-Pla­ne Se­mi­con­duc­tor La­sers XV, 97670O (2016)
  • [4] R. H. Pilny, B. Döpke, C. Bren­ner, J. C. Bal­zer, and M. R. Hof­mann, “Op­ti­miza­t­i­on of a mo­de-lo­cked diode laser by ma­ni­pu­la­ti­on of in­tra­ca­vi­ty dis­per­si­on and ab­sorp­ti­on with an evo­lu­tio­na­ry al­go­rithm”, R. H. Pilny, B. Döpke, C. Bren­ner, J. C. Bal­zer, and M. R. Hof­mann, CLEO Eu­ro­pe (2015)
  • [5] J. C. Bal­zer, R. H. Pilny, B. Döpke, A. Klehr, G. Er­bert, G. Tränk­le, C. Bren­ner, M. R. Hof­mann, “Pas­si­ve­ly Mo­de-Lo­cked Diode Laser With Op­ti­mi­zed Dis­per­si­on Ma­nage­ment”, IEEE STQE, Vo­lu­me 21, Issue 6, 1101008 (2015)
  • [6] B. Döpke, R. H. Pilny, C. Bren­ner, A. Klehr, G. Er­bert, G. Tränk­le, J. C. Bal­zer, and M. R. Hof­mann, “Self-op­ti­mi­zing fem­to­se­cond se­mi­con­duc­tor laser”, Op­tics Ex­press Vol. 23, Issue 8, pp. 9710-9716 (2015)
  • [7] B. Döpke, J. C. Bal­zer, R. H. Pilny, C. Bren­ner, A. Klehr, G. Er­bert, G. Tränk­le, M. R. Hof­mann, “Ul­tras­hort pulse ge­ne­ra­ti­on with se­mi­con­duc­tor la­sers using in­tra­ca­vi­ty pha­se- and am­pli­tu­de pulse shaping”, SPIE 9382, Novel In-Pla­ne Se­mi­con­duc­tor La­sers XIV, 93820D (2015)
  • [8] J. C. Bal­zer, B. Döpke, C. Bren­ner, A. Klehr, G. Er­bert, G. Tränk­le, and M. R. Hof­mann, “Mo­de-lo­cked se­mi­con­duc­tor laser sys­tem with in­tra­ca­vi­ty spa­ti­al light mo­du­la­tor for li­ne­ar and non­line­ar dis­per­si­on ma­nage­ment”, Op­tics Ex­press Vol. 22, Issue 15, pp. 18093-18100 (2014)
  • [9] J. C. Bal­zer, B. Döpke, A. Klehr, G. Er­bert, G. Tränk­le, M. R. Hof­mann, Fem­to­se­cond se­mi­con­duc­tor laser sys­tem with ar­bi­tra­ry in­tra­ca­vi­ty phase and am­pli­tu­de ma­ni­pu­la­ti­on, SPIE 9002, Novel In-Pla­ne Se­mi­con­duc­tor La­sers XIII, 90020D (2014)

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